Imam Ali ibn Abu Talib's merciful behaviour towards his killer:
It was the month of Ramadan, the month of fasting. It was the time of the morning prayers; the Kufa mosque was full of Muslims. One man called as Ibn Muljim, was pretended to be sleeping with his face downward. He had hidden a poisonous sword under him. Imam Ali (as) was kneeling before Allah (SWT) and when he raised his head a terrible blow fell upon him giving a very deep cut from the poisonous sword of Ibn Muljim. There was a great disturbance and commotion in the Kufa mosque. The murderer Ibn Muljim started running. The Muslims followed, caught and bound him in ropes and brought him before Imam Ali (as) who was on the prayer carpet drenched in blood and was reclining upon his sons. Imam Ali (as) knew the blow was fatal and he could not survive it but when the murderer Ibn Muljim was brought before him, he saw that the rope which had bound him was so tightly bound that it was cutting into his flesh. Imam Ali (as) turned towards those Muslims and said:
You should not be so cruel with your fellow being slacken his ropes, do not you see that they are cutting into his flesh and he is in agony.
Ibn Muljim was thirsty, that Imam Ali (as) could see. Then Imam Ali (as) was taken to his house. There was much crying and weeping in whole Kufa. At this time milk was brought for Imam Ali (as). He looked at the milk, and said, “Give a glass like this one to Ibn Muljim also, he is very thirsty.”
The milk was given to the murderer Ibn Muljim. He gratefully drank and quenched his thirst.
So was Imam Ali ibn Abu Talib (as). History of Islam is full of the incidents of his chivalrous and merciful behaviour.