Hmm... The cost of killing them installing vs or keeping in a cell where the cost builds up and up. Also you have to remember we aren't executing people everyday(unless you live in Texas). You also have to think about the prison population because you can only hold so many. If you live in California you will know that we have more prisons than schools and we are still building more. Eventually we released many minor offenders to make room for the more violent offenders. Also some sick guy is going to profit off this by siphoning tax money used for these prisoners, privately owned prisons scandals(cash for kids scandal any one?).
So what I trying to say is that we should avoid System Corruption, Prison Overcrowding, and a police state.