For most of history you would be better off if you could kill the next village over. You want to be friends with the people in your village, but if you kill the next one you can expand your farm/hunting/gathering grounds and then leave it to your kids - while otherwise you won't have enough food for all the kids and your DNA is in danger of not getting passed on.
In our modern world we mostly have plenty of food (and when we don't lack of land is not the issue), but that isn't what our DNA is evolved to "think"