Something I've never understood: sadism. Gaining pleasure from causing pain, in any form, to others.
It's somewhat comprehensible, if no less reprehensible, when someone does something at a gain to onesself that hurts another. It's a Machiavellian tendency: the end justifies the means, no matter who you kill along the way. It's a rather seductive line of thought: do whatever it takes and it's okay if it turns out right in the end.
The problem is that "right" is a very subjective thing, and whatever you've done along the way is part of the end.
What I truly can't understand, however, is those who cause pain for the entire purpose of causing pain. How can one possibly derive joy from the suffering of another? Why is there such a widespread epidemic of people hurting others- any sort of "others," be it person, place, or thing- and enjoying it?
It's incomprehensible. I take joy in helping others: I like to see that, from my action, I'm making life better for someone. Any someone will do. And it makes sense to me: I understand that something I've done has made the world, overall, a slightly bettre place, and that's a good feeling. To know that I've made a positive difference.
Why do some take the same pride and joy in knowing that they've made a negative difference?
There is also a lot of, well, trust involved in S&M pairings, and for those who aren't used to it this can be an unusually strong thing. Again, it relates back to the power issues.
...At least, this is my take on it.
It might be related to schadenfreude. In fact, one might say that sadism is a bit of an extreme and pro-active form of schadenfreude.