Part of the reason I dislike many evo-psych explanations as an archaeologist is that they end up reflecting a lot of the author's assumptions about the nature of human societies and not the diversity of forms we actually observe. You can support virtually any argument by just narrowing or widening this scope appropriately. One example here is that young women are free to actually decide which "suitors" to accept. In some gerontocracies, young women generally don't have that choice, socially at least.

This is just speculation on my part, but maybe being cut-throat is the strategy -- it's just applied to both potential suitors and potential competition. If you're at the top of the pyramid then the easiest way to discourage both unsuitable competition and unsuitable suitors is to be mean about it.

Now that I think about it a little more, it's probably more applicable to be mean to anyone in an outgroup.

And now that I think about it even more, I really have no idea of what I'm talking about. Most of the time that I've been mean to people, when I was younger, it was basically because I was an immature and confused jackass. And, as far as relationships go, who hasn't made a complete ass of theselves at one point or another?

What if you're second best? What if you don't know your position?

Game theory.

when 'best' is subjective, perceptions and biases come into play...

But when you are in a competition even if you are nr. 1 you still have to defend the position so you still need to cut some throats.