While people are of course more alike to each other than, for example humans and cows, there are still a lot of differences between single persons. People tend to assume that other think as they do, but that's one of the most common sources for misunderstandings.

There are people who can torture a child to death without feeling the smallest amount of guild (maybe they even find it funny or satisfying), others who could do it with the right amount of indoctrination, others who would only do it if their life is in danger (and would feel horrible afterwards) and others who would rather die than doing it.

And it's not possible to shift a person people into all of those categories, it's mostly inborn (like sexual orientation, hair color, etc).

Sure, all those people have lots of other "basic imperatives" in common, but the small difference that some could inflict as much pain as possible to others without feeling even a little bit guilty is a difference with should have quite severe consequences for society as a whole.

So if we try to understand why there is so much violence in the world, it would be quite an oversight to not take those kind of differences into account.