Well, that is the biological norm.

And better than friend vs stranger ;)

Any objective justification for that or just your own bias?

Just as mirimir said, it is biological norm to draw line between your own kind and everyone else. For example, both dogs and cats don't kill each other, but dog can easily kill a cat.

Chimps form tribes and go to war. https://en.wikipedia.org/wiki/Gombe_Chimpanzee_War

Both dogs and cats do kill each other. It is not uncommon at all.

Yep, cats fight brutally all the time. My wife's old cat was a fighter and had the scars to prove it. No doubt he had killed many cats, and then he eventually succumbed to an infection from a nasty neck wound acquired in a fight.

Ha ha ha, he was quite champion

Just because I'm human.

Are you a mammal?

Especially if we're adrift and starving. In that situation, the more complex the nervous system, the better.

Why limit yourself? Draw the line between you and not-you. Such fun.