For parents, that could be possibly explained by the parent being abused by their own parents: that behavior is acceptable, or they think they're being "good parents" because they aren't beating you with a hockey stick. And also, the parent could have a pre-conceived notion of what that child should be (athletic, or a certain GPA, prettier, you name it) and blame it on the child.
Of course, then there's plain old status, too. Especially if the abuser is being put down themselves. They might feel the need to exert power in the only capacity that they know how.
I think this is the best answer so far..
For a more complete answer, I would add something about an abused person that has been unable to heal wants to recreate the negative life situations with those closest and most dependent, so they don't feel so isolated, to justify their experiences as 'normal' and also out of spite/anger at the world for not treating them properly.
Of course, abuse of dependents and 'malice' are different things requiring two different explanations, though not mutually exclusive. I suppose malice is more about increasing or experiencing (larger group) 'status' as the other comments have been focusing on.
The other day, someone did something I didn't expect. It was something many people have done before; something that I thought of as very normal, but that I in no way understood and had not predicted.
As I said, this had happened many time before, so I wrote it off as "me not understanding people" or "people are weird" for a second, like I usually do, before realizing that "bad at" really means "lacking basic knowledge", which I had never realized before.
And then I thought "I should ask someone who is different from me why people do that, and eventually someone will have an answer."
But many people will have many more questions like this. So, what have you observed people doing time and time again, but never understood? Or something that you only understood after a long time or asking someone about it?
And can Less Wrong tell us, not necessarily why (I for one can make up evolutionary psychology fairy tales all day if I want) but what conscious thought process occurs behind these events?