To what extent is the will of Imam Ali to his son Al-Hasan (may Allah be pleased with them) authentic, concerning the treatment of the killer, which includes being kind to him and then seeking retribution from him, and other wills related to piety and working for the hereafter?

The will of Ali ibn Abi Talib, may Allah be pleased with him, concerning his killer is well-known and agreed upon by biographers. He commanded that his killer be executed in retribution (qisas) and forbade mutilation of his body. Among what was narrated from him is that he said: "Feed him and give him drink, treat his captivity well. If I live, I am the guardian of my blood; I may forgive if I wish, or I may seek retribution if I wish. But if I die, and you kill him, do not mutilate him." It was also narrated from him: "If I survive, I will decide his fate. But if I perish from this blow of mine, then strike him one blow, and do not mutilate him, for I heard the Messenger of Allah, peace and blessings be upon him, forbidding mutilation, even of a rabid dog." As for the narration that includes his command to burn him, its chain of transmission is weak.