Sure. But the question is why? Why is causing unnecessary h…

Sure. But the question is why? Why is causing unnecessary harm not good?
We are neck deep in religious ideas and therefore we just assume it is obvious when it really isn't at all.
Causing unnecessary harm gets you riches, women and helps you spread your DNA.
How is Genghis Khan not good? He was the best!
There is, of course, a difference to be made between ingroup and outgroup. You cooperate with the ingroup and destroy the outgroup and take their resources and women.
In an atheist/materialist context saying "that is evil" is nothing but an admission of weakness (slave morality). "I don't like it because I'm on the wrong end of it."
Killing is great for the killer, but bad for the victim.
The only way out of this is metaphysical.