If I had met another hominid on my way, I doubt I would want to kill it right away because it horrifies me. I would rather expect the natural trigger that prevents me from killing other people to turn on.

I'm not saying that different species of hominids didn't kill each other, I just think that the main reason behind killings was fight for food and other resources, not the low level fear instinct that we experience during encounters with spiders, snakes and worms.