Allah, the Wise, says:
عَسىَ أَنْ تُحِبُّوا شَيْئاً وَ هُوَ شَرٌّ لَكُمْ
"It may be that you love a thing while it is evil for you" 
Imam Sadiq (a.s.) has said:
وَ إِنَّ الْعَمَلَ السَّيِّئَ أَسْرَعُ فِي صَاحِبِهِ مِنَ السِّكِّينِ فِي اللَّحْمِ
"Surely, the effect of an evil deed upon the doer is faster than that of a knife upon a piece of meat." 
The worst of people is one, who sells his hereafter in exchange for his worldly life, but worse still is one, who sells his hereafter for the worldly lives of the others.
Evilness manifests itself in numerous forms, all of which could be summarized as disobedience to God.
Since deeds are subordinate to intentions, evil thoughts generate evil deeds. When a person does not have God within his sights, he considers himself to be strong and powerful. He indulges in various acts of evil, without his heart experiencing the slightest fear of Hell.
For each limb and organ of the body there are evil actions which are associated with and specific to them; the ears to backbiting, the eyes to seeing prohibited things, the tongue to lying and the hands to hurting the orphans. It is therefore essential to preserve all of them from evil.
1 – Misfortune of Jaludi
After the martyrdom of Imam Kadhim (a.s.), Haroon Al-Rashid, the Abbasid Caliph dispatched one of his commanders, a person by the name of Jaludi, to Madinah and instructed him: "Attack the houses of the progeny of Abu Talib, loot the womenfolk and leave behind nothing, except one apparel for each of them!"
Once in Madinah, Jaludi started to execute Haroon’s orders. As he neared the house of Imam Ridha (a.s.), the Imam (a.s.) gathered all the ladies of the house into one room and stood at the door, preventing Jaludi from entering.
Jaludi insisted that he must enter the house and loot the ladies and take away their apparels. The Imam (a.s.) promised that he himself would collect their clothes and ornaments and hand them over to Jaludi, on the condition that he should abstain from entering the room.
Jaludi eventually acquiesced to the Imam’s request, whereupon he entered the room, gathered the ornaments and clothes of the ladies and, together with the other things of the house, placed them at Jaludi’s disposal, which he promptly dispatched to Haroon.
After Haroon, it was his son Mamun, who took over the reins of the Caliphate. It so happened that one day he became angry with Jaludi and sought to punish him with death. Imam Ridha (a.s.) had been present in that assembly and requested Mamun to forgive him.
Jaludi, recollecting his previous wickedness with respect to the Imam, thought that he (a.s.) would complain about it to Mamun and so, turning to Mamun, he said:
"I place you under the oath of God! Do not accept his words in connection with me."
Mamun said: "By God! I shall not accept his words." Saying this, he then ordered Jaludi to be beheaded.
2 – Deceit by A’mr Ibn A’as
After the incident of arbitration, in which A’mr Ibn A'as tricked Abu Musa Asha’ri and deposed A'li (a.s.) from the caliphate, the Imam (a.s.) used to curse him, Mua'wiyah and Abu Musa after the morning and Maghrib prayers.
A’mr Ibn A'as was also part of the group that was involved in the incident of the night of A’qabah, and had subsequently been cursed by the Holy Prophet (peace be upon him and his holy progeny) too.
When the dispute between Imam A'li (a.s.) and Mua'wiyah intensified, it was decided that the issue should be resolved by means of arbitration. Unfortunately, the people of Iraq selected Abu Musa Asha’ri to represent the Imam (although, the Imam himself was not happy at this selection), whereas Mua'wiyah decided on A’mr Ibn A'as as his representative.
Abu Musa, who was in one of the villages of Shaam, was asked to present himself in Siffeen and four hundred people, amongst them Shuraih Ibn Haani and Ibn A'bbas, accompanied him to Daumah al-Jundal. A’mr Ibn A'as also arrived there with four hundred of his companions.
All the counselling and recommendations provided to Abu Musa proved futile since A’mr Ibn A'as, with the evilness of intention and wickedness of character that he possessed, was far more powerful than him in shrewdness and deception.
One of A’mr Ibn A'as’s techniques was to exhibit exaggerated deference towards Abu Musa. He sat him in the front of gatherings and insisted that he lead the prayers, while Amr himself prayed behind him, and all the while addressing him as O’ Companion of the Prophet of God! He used to say to him: "You have had a precedence over me with regards to the companionship of the Holy Prophet (s.a.w.) and are elder to me and thus, it is unbecoming of me to speak on something before you have done so".
He presented such an elaborate display of respect that the simple-minded Abu Musa was convinced of his uprightness and became certain that his only intention was to set aright the existing state of affairs. As part of his cunning plan, A’mr Ibn A'as initially took Abu Musa to a secluded place and spoke to him in solitude so as to prevent others from influencing Abu Musa in making a decision.
A’mr Ibn A'as asked him, "Abu Musa, what is your opinion regarding A'li (a.s.) and Mua'wiyah?"
"Let us depose A'li (a.s.) and Mua'wiyah from the caliphate and allow the issue of caliphate to be managed by a Counsel," responded Abu Musa.
Hearing this A’mr Ibn A'as said, "By God! Your opinion is absolutely correct and we must put it into execution."
After agreeing to this course of action, they came out in public.
Abu Musa rose up first and began to speak when Ibn A'bbas cried out, "Be wary, for I fear A’mr Ibn A'as has tricked you. Allow him to speak before you."
But Abu Musa paid no heed and said, "O’ People! A’mr Ibn A'as and I remove A'li and Mua'wiyah from the caliphate and shall (only) approve of a caliph selected by means of a Counsel. I hereby remove A'li (a.s.) from the caliphate."
Then, the wicked A’mr Ibn A'as stood up and said, "I too remove A'li from the caliphate but appoint Mua'wiyah in his place; Mua’wiyah seeks to avenge U'thman’s death and is therefore most deserving of this rank."
"You are like a dog," Abu Musa shrieked out, "that attacks if one approaches it and does the same if one turns away from it."
A’mr Ibn A'as retaliated, "And you are like a donkey, which carries a hoard of books (but does not benefit from them in the least)."
In short, A’mr Ibn A'as, supported by his evilness, emerged the victor in the issue of arbitration! Later, Ibn A'bbas used to say: "May God disgrace Abu Musa! I had warned him of the guiles and the evil intentions of A’mr Ibn A'as and advised him rightly, but he turned a deaf ear and refused to take heed."
3 – Cruelty of Hajjaj ibn Yusuf Thaqafi
It is not just evil deeds which merit chastisement but evil intentions too tend to have an impact. In fact it is due to their evil intentions that the disbelievers and the enemies (of Islam) shall reside in Hell eternally.
Hajjaaj Ibn Yusuf Thaqafi used to exhibit great cruelty and evil by imprisoning and killing the saadaat (descendants of the Holy Prophet (s.a.w.)).
Once, while coming out of the mosque and hearing the wailing and crying of a great number of people, he asked, "Who are these who wail?"
Those around him said, "These are the wailings of the captives, who are tormented due to the intense heat of the sun."
He said, " Tell themاِخسئُوا(scram away),"which, in the Arabic language, is also employed for driving away a dog.
His prison contained 120,000 males and 20,000 females (4,000 of the females being single) and was one large area, walled but roofless. Each time the prisoners tried to shelter themselves from the scorching sun, either with their hands or some other means, the guards overlooking them would strike them with stones.
Their food was bread, made of barley and mixed with sand, while their drink was bitter water. At times the blood of the saadaat and the righteous ones would be utilized for preparing Hajjaaj’s bread, which he would eat with great relish!
This wicked person always regretted not having been in Kerbala and used to say, "O’ How I wish I had been in Kerbala so that I could have had a hand in killing Imam Husain (a.s.) and his companions!"
4 – Justifying Evil Acts
Imam Sadiq (a.s.) had heard that an old man had become famous for his piety. One day, he saw him surrounded by a large crowd. A little later, the man came out of the crowd and distancing himself from them, proceeded alone, whereupon the Imam (a.s.) began to follow him. After a short time, the Imam (a.s.) observed that he had stopped near a bakery from where he stealthily picked up two loaves of bread. After a short distance, he stopped at a fruit store, picking up two pomegranates in the same manner and once again continued on his way.
As he walked further, the old man approached a sick person, handed over the loaves and fruits to him and was about to move on when Imam Sadiq (a.s.) came up to him and said, "I have witnessed something greatly astonishing from you", and then proceeded to narrate the acts, which he had witnessed.
The old man said, "I suppose you are Imam Sadiq (a.s.)."
The Imam (a.s.) replied in the affirmative.
The man continued, "It is really unfortunate that in spite of being of the progeny of the Holy Prophet (s.a.w.) you do not seem to know anything."
The Imam (a.s.) asked, "What act of ignorance have you noticed from me?"
The man said, "But do you not know that God has said in the Qur'an "Whoever brings a good deed, he shall have ten like it and whoever brings an evil deed, he shall be recompensed only with the like of it". 
On this basis, since I have stolen two fruits and two loaves of bread, I have four sins in my account, but on the other hand, since I have given it in the way of God, I have earned forty good deeds. Reducing four from forty, I still have thirty-six good deeds in my account; a pity that you possess no knowledge of such computations!"
The Imam (a.s.) explained to him, "But have you not heard this verse of the Qur'an, which says: "Allah only accepts from those who guard (against evil)".  You have earned four sins by stealing those four items and four more sins for giving them tosomeone else without the permission of the owners, so you have collected eight sins but not a single good deed."
Later, the Imam (a.s.) said to his companions, "With such interpretations and justifications, not only do they mislead themselves, but others as well."
5 – The Consequence of Evil Deeds in Barzakh
A distinguished scholar, renowned for his piety, narrates:
One of my relatives had purchased a property during the last years of his life, utilizing the abundant income derived from it, for fulfilling his needs.
After he died, I witnessed him in the purgatory, in a state of blindness. When I asked him reason for it, he replied:
"I had purchased a piece of land, in the centre of which existed a well, whose water was utilized by the inhabitants of a nearby village for themselves and their animals. But their passage through my land used to damage a portion of my crops and so, in order to protect my income and to prevent them from coming in, I filled up the well by means of sand and stones, and covered it up. As a result, the hapless inhabitants were forced to travel a great distance to procure their water, and this blindness is an outcome of that act of mine."
I asked him, "Is there any solution to this problem of yours?"
He replied, "If my heirs were to exhibit mercy upon me and uncover the well so that the others benefit from its water again, I shall come out of this predicament of mine."
Approaching his heirs, I informed them of the incident. They agreed to do the needful and very soon, the well was opened up and the people began to make use of it, as before.
After a period, I again witnessed the deceased, but this time noticed that his sight had been restored and he was thankful to me for helping him come out of his misery.
 Holy Qur'an, ch. Al-Baqarah (2), vs. 216.
 Jaame’ al-Sa’adaat, vol. 3, pg. 48.
 Raahnama-e-Sa’adat, vol. 1, pg. 177; Aa’yaan al-Shia’h, vol. 1, pg. 60.
 After the Holy Prophet (peace be upon him and his holy progeny) had proclaimed A'li (peace be upon him) his successor at Ghadeer Khum, some individuals gathered together and conspired to conceal themselves near a mountain path and startle the camel of the Holy Prophet (peace be upon him and his holy progeny) in order that he is hurled to the ground and killed, and in this manner sought to prevent the caliphate of A'li (peace be upon him) from being established in Madinah, and this is the night, which is referred to as the ‘night of ‘Aqabah’
 Paighambar Wa Yaaraan, vol. 1, pg. 139 – 153; Bihaar al-Anwaar, vol. 8, pg. 544.
 Holy Qur'an, ch. Al-Mu’minun (23), vs. 108: (اخْسَؤُا فِيها وَ لا تُكَلِّمُونِ ) - an address to the inmates of Hell
 Pand-e-Taareekh, vol. 3, pg. 163; Raudhaat al-Jannaat, pg. 133.
 Holy Qur'an, ch. Al-An’Am (6), vs. 160: (مَنْ جاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثالِها وَ مَنْ جاءَ بِالسَّيِّئَةِ فَلا يُجْزى إِلاَّ مِثْلَها)
 Ibid, ch. Al-Maidah (5), vs. 27: (إِنَّما يَتَقَبَّلُ اللَّهُ مِنَ الْمُتَّقِينَ )
 Namunah-e-Ma’arif, vol. 4, pg. 275; Wasaail al-Shia'h, vol. 2, pg. 57.
 Daastaan-ha-e-Shigift, pg. 292.