The set of comments to this post are what I consider another example of the relative social(?) weaknesses the typical population of Less Wrong has compared to the general population.
If this question was asked of a college humanities class, a subset of the answers would be much better than these and the class as a whole would be better at identifying the answers that are more correct.
There are some answers below that are fine, but they're not said with enough confidence, and the 'ev psych' answers are not reliable. Not because ev psych can't hit upon an answer that might be correct, but because it seems that those that rely on them are not able to compare the hypothesis with a lifetime of experience for general plausibility. [On second thought, I retract this last comment to the extent people were explaining petty malice, which was indeed the first and main question of the initial post, rather than abuse of dependents. The difference being the extent to which there is absent verses perverted empathy.] I still think the answers would be better if we quizzed a random population of college students -- the academic setting just to avoid answers like 'people are evil'.
I don't respond to this question because formulating any kind of reductionist answer frames my perspective in a way that is unsettling. However, I'd be happy to identify a more correct answer when I see it. Psychology, generally, is the aspect of science where we've made the least progress with respect to reductionist explanations.
Does anyone have a college humanities class that we can ask? I'm betting we would get some replies to the effect that "people are evil", but I'd be interested in whether the average is generally better.
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?