I think it's worth remembering that the people who do this sort of thing, especially to random strangers, are not usual. Most people never beat anyone at all to death, so when it happens, whomever it is, there is something about the perpetrator or situation that is different from what happens to most people. So that's where we have to look for an explanation, mainly.
I do think these cases where one person does this, there is at least some kind of rage or hatred involved, that is beyond the norm. Possibly also a lack of self-control, which may be something in the person or could be caused by something external like drugs or booze. Booze is implicated in a lot of brutal violent crimes.
Are there rational but evil people who just decide they are going out to beat some woman to death? Maybe, but it's a pretty risky thing to do no matter how smart you are. I suppose the cases where it happens are where you get serial killers who plan their exploits, but that is a much smaller group I'd think.
When you have groups of men who do stuff like this, I think there can be more complex things involved, people can do terrible things in a group that they would never do alone.