How can a person who caused harm to his parents and incurred their curses upon himself, wishing him death, act after the death of his mother, who died because of him, and with his father on the verge of death and displeased with him? Will his supplications for mercy and forgiveness for them be answered? And is he considered a murderer of his mother, thus deprived of inheritance?

The rights of parents are immense. Allah has enjoined their rights immediately after His Own, and the Prophet, peace and blessings be upon him, warned against disobeying them, considering it one of the gravest sins. A disobedient child must repent sincerely and hasten to honor their living parent through kindness. They should strive to honor their deceased mother by supplicating for her, seeking forgiveness for her, fulfilling her pledges, honoring her friends, maintaining kinship ties through her, and giving charity on her behalf. As for what you mentioned about his mother's death being caused by him, this is not sufficient for a fatwa without further clarification.