How morally different are ISIS fighters from us?

Uhm, taboo "morally different"?

Are their memes repulsive to me? Yes, they are.

Do they have terminal value as humans (ignoring their instrumental value)? Yes, they do.

How about their instrumental value? Uhm, probably negative, since they seem to spend a lot of time killing other humans.

If we had a similar upbringing would we think it morally correct to kill Yazidi children for having the "wrong" religion? Or might genetics play a role in our differing moral views?

Probably yes. I think there can be a genetic influence, but there is much more of "monkey see, monkey do" in humans.