Because they were depressed and had a litany of other issues. Since it was taboo to talk about (and confront) those issues they developed fucked up coping mechanisms. Like hitting their kids/spouse. Explosive tempers, or just being an asshole. Drinking, gambling, cheating, etc.
Two of my grandparents were hardcore alcoholics that drank themselves to death after attempting suicide multiple times (which I only heard about as an adult). The third succeeded in hanging themself. The fourth lived to old age and was one of the nastiest, most narcissistic people I’ve met.