It can and has been argued that aggression has served a variety of evolutionary purposes. However, it no longer seems to be of any benefit to our species in today's world. We also don't seem to be able to prevent it.
Another scenario I would accept as proof of God's existence is the cessation of all forms of human aggression. Since it no longer seems to be of benefit to us and we can't seem to end it ourselves there is no reason God could not solve the problem for us. I am fully aware of the rationale many theist use to contradict this idea. Free will has always been a pathetic excuse. I do not know a single person who has chosen to be mugged or asked for children to shoot other children. Since, I and others routinely push for crime prevention measures and gun control why are we made to suffer along with those who refuse do do anything useful about these problems. If God created us and our world then there is no reason God cannot make such a modification as the elimination of all aggression. There would still be plenty for us to disagree and make decisions on. So assuming that free will is even possibly in the presence of a supreme being, which it isn't, it would be unaffected by the absence of aggression.