Male lions kill the cubs of other males whenever they can. There is no discernable reason, really. They just hate them. This behaviour serves to diminish the spread of rival genes, and therefore it makes evolutionary sense. Random killings can serve to weed out the weak in a population. That is natural selection at work.
If one of our ancestor species ever developed (by natural selection) this urge to kill, it would automatically carry on, because only the random killers kill for no reason, the others live and let live which does nothing to weed out the weak. Thus, random killing, if it came up, which it certainly did, must have stuck with us.
Today it must seem like a ghost from the past, atavistic even, but the process of natural selection will never go away, until all species are extinct. So don’t worry if you feel an urge to kill. It is nothing sick. It is healthy! It is the logical result of millions of years of evolution, and would serve even today to accomplilsh the same thing it did for millions of years. Besides, we have many instincts that keep us from acting out these urges in real life (and even in dreams, to which I can testify).
When it comes to curiosity, it is a natural thing, but what you are curious about depends on influences. So your upbringing has a hand in it. There is probably a complicated mix between influences and natural fascinations, that leads to a desire to, punch someone to see what happens. It is interesting to see how the person reacts, but it is even more interesting to see how one reacts oneself. You want to know if you are that brute, or can become that brute. Ofcourse aggression comes from the “reptile brain”, so, it’s even deeper than cave man behaviour. Imagine yourself as a lizard. This kind of exploration is perfect for dreams. How else can you tap into that lizard brain of yours and try to ignore “higher” functions, and where else it is at all acceptable?
If you can’t go nuts in your dreams, you can’t go nuts anywhere!!! Go ahead! Be violent if you wish! Be selfish! Why not? That kind of liberating experience of just going with impulses, no matter how crazy they would be in real life, just can’t be found anywhere else! Cutting off your leg with a chainsaw is a perfect example.