America was founded on the ideals of religious freedom by people who were predominantly Christians, this led to Christianity being the 'popular' religion of this country, but it is in no way the only religion.
Unfortunately that has led to as many problems for Christians as it has solved. Since Christianity is the 'popular' religion many people say they are a Christian when they really aren't (i.e. 'Sunday Christians'). This has led to many incidents where people who claim to be Christians do something terrible which then gets blamed on Christianity as a whole instead of the individual, this is especially disheartening when their actions go against Jesus' teachings.
Think of it like sports fans who only follow the team that is currently winning, and as soon as another team comes out on top they switch over and are running around with a whole new set of colors and jerseys. Meanwhile they are starting fights and causing disturbances all of which get blamed on the 'fans' of the team whose jersey they happen to be wearing at that time.
As for the original sentiment, I agree that as 'punishment' death is far too excessive for assault and armed robbery, however as 'Karma' it works just fine.