Why was the earth full of violence before the flood?
Violence filled the earth before the flood because there was no deterrent, no capital punishment, and no law against such things. God had not yet given man responsibility to restrain murder through human government. Only after the flood did He institute capital punishment and human government to maintain law and order.
Les teaches that after the flood God gave Noah and his sons a whole different set of circumstances. In Genesis 9:5 God assigns man the responsibility to keep the human race from killing each other, as they had just been doing. Violence had filled the earth before the flood, and it was simply because there was no deterrent, no capital punishment, and no law against those things. So God instituted human government or capital punishment to maintain law and order.