Some people (like abuse or neglect victims) have a damn good reason for hating their parents.

Hurt people hurt people. These parents were, very likely, the victims of abuse and neglect during their childhoods. As their parents were in turn. That's before you get into the impact of shit like lead poisoning on the human brain. The whole 80s-era drug/crime wave can be traced back to lead gas and lead paint. The damage done to people's brains caused all sorts of mental illnesses.

Even setting aside all of that, lets say your parents had no excuse. Lets say they're straight up Jeffery Epstein grade sociopaths who deserve every ounce of hate you can fling their way. It's still just, like, not good for your own psyche to dwell on these people. Trauma isn't something you should seek to cultivate in yourself. It's poison. It's debilitating and agony-inducing. Escaping that agony is far better for you, if you can manage it. And if you can't, that's what good comrades are supposed to help you out with.

The goal of Leftism isn't to exact revenge on the old world. It's to purge the poison from the system so that the next generation has it better than we did.