Instinct covers a lot of basic territories; eat when hungry, fuck when horny, sleep when tired, fight when threatened. That's all a human needs at the most basic.
Instinct is malleable through mutations in DNA, so nothing is necessarily "ingrained", but instinct also allows new sensory input to cause the human to deviate from basic instinctual reactions.
So, to kill another human could be an instinctual reaction if we were threatened by them. We can also run into many other reasons to kill another human and our instinct will just step aside and let the rest of our brains take over because it trusts the brain to learn, experiment, and make logical decisions about how to reach a goal. Thus the ratiionale for wars and sporting fights and duels.