Not Always Malicious
A lot of people assume that parents who cause lasting psychological harm to their children are just simply bad parents. Either they, for some reason, want their children to suffer, or they just don't care. Either way, they're bad people.
But in reality, it's not nearly that simple. Maybe there are some parents who really, truly don't love their children, but they're much less common than you might think. Most parents who hurt their kids actually love those kids and want the best for them, just like the more successful parents - but somehow, something goes wrong.
Maybe they're faced with a problem they don't know how to deal with. It could be that they think their children need something when they really need something else, like a parent who honestly doesn't know that shaking a baby can cause brain damage, or that 5 year olds can't be left unsupervised all day. Or they hold certain stereotypes about a group of people, and then suddenly discover that their child is a member of that group. Maybe they think one thing is going on with their child, and then find out that they're wrong and the consequences of their mistakes are serious.
Maybe they put their children in a situation without understanding what was really going on there. Who would have guessed that it's not all right to leave the boy with Uncle Ernie? Parents don't know everything. And they might not recognize the signs, when their kid is trying to tell them that something's wrong. Or maybe the child isn't trying to tell them - xe thinks they know already, or can't/won't help, or something terrible will happen if xe doesn't just put up with it, or maybe the child doesn't realize what's happening to xe is wrong.
Or maybe the parents have their own problems, and these problems are so serious and hard to deal with, leaving them without the resources they need to be good parents. Maybe their emotions keep bubbling up out of control, and when the child does some minor little thing, they can't restrain themselves and they hurt the child even though they know it's wrong. And they're sorry afterwards and they resolve not to do it again, but as long as they don't deal with the underlying problem, it will happen again. And maybe they don't know how to seek help, or they think they can handle it, or they're too afraid to seek help, or there's just no help to be found. So they're caught in this vicious cycle, and they can't find the way out. And the more often it happens, the more ingrained it becomes, until only something drastic can stop it.