...wars are always about population pressure and competition for resources. Always. Your virus could poop Valium and morphine -- but it wouldn't change the root cause.
You're begging the question. There is no 'be violent' chemical in our brains. It's a complex emergent behavior. So your hypothetical virus would either be ineffective, or so effective it make the organism (that's YOU) unable to feed itself.
Decreasing our testosterone would reduce violent tendancies, but it would also mess with our ability to reproduce. And besides, war isn't usually about irrational aggression, it's often more about fear, which afaik is mitigated by testosterone
So what would happen if war is never an option?