How many cultures and societies have believed in gods, a creator, supernatural entities, etc, yet were violent? History has many examples.
You are assuming that violence corresponds with immorality. You are also assuming that an atheist society would be different.
Violence may arise for any number of reasons, scarcity of resources, stratification, power in the hands of a few, royalty, kings, emperors, political ideology, religious hierarchy...look at the power of the church during the middle ages.
