They are the foundations of every civilization, but it's not like we suddenly emerged and killed one another. We don't want to murder one another. That's why we get things like PTSD - not always from murder, it can be a lot of things adding up to it - when we murder and kill one another.
It just isn't us. Yet, we certainly have it within us to do so... like you said: every civilization mankind had since the agricultural revolution was founded and maintained thanks to bloody violence.