(Part No. 1; Page No. 734)
a Muslim dies after deliberately killing a person or persons, then he has committed a major sin other than Shirk (associating others with Allah in His Divinity or worship). He is left to the Will of Allah Who forgives or tortures him according to His Will. Allah (Exalted be He) says:
Verily, Allâh forgives not that partners should be set up with Him (in worship), but He forgives except that (anything else) to whom He wills
Then on the Day of Judgment, there will be a sort of settlement between him and his victim regarding misdeeds and good deeds. You only have to ask Allah to forgive your father and be merciful to him. You can also give Sadaqah (voluntary charity) on his behalf so that Allah may have mercy on him and pardon him.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.