It doesn't just begin, it advances. Thanks to the process of evolution by natural selection, violent behaviour develops over the ages - of course, only the most violent victors get to pass on their genes, while pacifists die alone. As humans develop more advanced tools, so more deadly become the violence.
Is that the case? Maybe pacifism survived because that trait avoided conflict (which most likely will result in death). But your ref here made it seem like that's not likely the case - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1266108/
Pacifism and social co-operation is an evolutionary advantage within clans. For intergroup violence, of course the more violent group survives.
That said, pacifism becomes an advantage much later, once diplomacy develops.