His father was always jealous of Marvin's talent. Plus he liked to wear women's clothes and act effeminate from time to time. Him and his father used to have big arguments and his father once said if Marv ever struck him he would kill him. Well, take things up to the 80s and you got Marv so addicted to cocaine and other drugs, plus depression, that he was suicidal. Threw himself out of a moving car at one point before the end, but he wasn't really injured. Then he gave his father a gun saying he was worried for him and his mother's safety. A few weeks later Marv's over at his parent's house and during an argument his father slaps his mother. Marv hits or pushes his father. His father goes to the other room and gets the gun and comes back and shoots Marv. Last heard, his father's in a mental hospital for old people claiming he doesn't remember a thing about it.
From the mother's statements, she knows better.
I believe suicidal Marv gave his father the gun knowing it would lead to him shooting Marv. Not only did he want to die at that time but he wanted to get even, in a way, with his father, who had always been jealous of him and kind of wanted him dead. He wanted his death on his father's conscience.