The moment animals ate animals 'violence' was born. All want to live, but none want to die......conflict is inevitable. When a thinking living creature survives by consuming/defeating another thinking living creature, for food or to procreate, there is the need for outsmarting snd overpowering another. No animal will subject itself to another without a good reason (some may for complex altruistic calculated reasons in the hope others may for their offspring/genetic releatives) and what is the alternative other than violence? Animals that survive in packs, colonies have developed social constraints as it is .......beneficial for the group and the individual!
Humans have developed reasoning as it is less risky than boxing, but every Saturday night will prove that we are still primitive!
Violence is a difficult term to pin down.... is it violent to kill a chicken as we do in the millions ..to make it into a KFC? Is it violent for a lion to take down a gazelle? What about one tribe dominating another for respect in order to dominate that region? What about two tribes ...separated by football shirts?
As a rule, asking "Why?" with regard to behaviour in humans and animals, the answer comes quite easily when pondering how the mechanism came about and what environmental factors led to that trait evolving.