Hazrat Salim (may Allah have mercy on him) says that once Sayyiduna Ibn Abbas (may Allah be pleased with them both) was asked about a person who killed a Muslim, then repented, embraced faith, performed good deeds, and remained steadfast on the path of guidance. He replied: Woe to you, how will he find guidance? I heard your Prophet (peace and blessings of Allah be upon him) saying that the murdered person will be brought in such a state that he will be clinging to his killer, and will say, "O Lord! Ask him for what crime did he kill me?" By Allah! Allah revealed this to your Prophet (peace and blessings of Allah be upon him), and after revealing it, He never abrogated it. Woe to you, how will he find guidance?

Note: This is the opinion of Sayyiduna Ibn Abbas (may Allah be pleased with them both). The majority of the Ummah agrees that if the killer, after repenting, adorns himself with faith and righteous deeds, his repentance is accepted. In the case of fulfilling the rights of people or punishment, in any case, due to the blessing of the Kalimah, he will definitely be saved from Hell at some point.