The puzzle isn't why people do things which involve hurting and killing other people and other creatures, as such. There are all kinds of reasons why that happens, and why people feel it right and/or necessary to do them. To protect themselves and others, for revenge, for food, for money, to power over othwer people, for the pleasure they find in exercising some skill they have acquired... Some of those reasons are pretty nasty reasons, but they are reasons.

The puzzle is to understand the kind of cruelty that doesn't fall under those kind of headings. And it isn't good enough to say that it's just the way people are, a natural human behaviour, because it's not the way most people are most of the time. Natural behaviour generally has some kind of function, and this seems not to, on the face of it. What takes us aback most perhaps is in fact that it feels un-natural. It has to have some kind of "function" from the people who do this stuff, and identifying what that is would be useful.