This one's actually going to be a little different to the others since I want to start off by talking about a film called The Green Mile. If you plan on watching it, you may want to stop reading unless you want it spoilt.
Tom Hanks' character, Paul Edgecomb, is basically in charge of Death Row and the title comes from the green strip of linoleum that is on the floor as prisoner's walk the 'last mile' before they are electrocuted. Michael Clarke Jordan's character, John Coffey, is one of those gentle giant types who ain't done nothin' to no one. Yet this guy is on death row for raping and killing two young girls. But, like I said, he ain't done nothin' to no one. As the film goes on, you learn that Coffey has supernatural powers which is where the film gets a bit weird but, as Edgecomb says, "I just can't see God putting a gift like that in the hands of a man who would kill a child." Coffey starts healing people and when William Wharton, another prisoner, arrives, he 'feels' that this Wild Bill guy is the one that actually did it. Basically, what my point is that Coffey is innocent. He never did anything but help people. Yet he was killed because the government believed it was their right to do so.
John Coffey's fictional but how many other innocent people do you think have died from the death penalty? Put them in prison for life, sure, because at least then if they are found innocent they still have part of their life left. And for those who really did the crime, life imprisonment is going to be worse for them then death. Plus, killing them for killing someone else isn't going to help. If you fight murder with murder, you'll end up with a massacre.