The reason for capital punishment, as the verses above in Genesis explain, is that man is made in the image of God. Murder is such a heinous crime in God's sight that the punishment is the loss of one's own life. The intent of the punishment is to raise the consciousness of the sacredness and value of human life. Instead of obeying God, the people chose to unify on the plain in the land of Shiner. God's intent was for the human race to spread out and form nations.
Nations are a check on evil.
Nations form competing power structures which help to eliminate concentration of power. At this time, everyone spoke one language. By concentrating in one area as one people there was a great potential to concentrate power and do evil. God has established nationalism to restrain evil and preserve freedom for man. World government is contrary to the plan of God and threatens that freedom.