An eye for an eye would only make the world blind.
If someone kills a murderer, aren't you as bad as them? You're commiting the same act, therefore it's a never ending cycle?
"There are only two important days in your life: the day you are born, and the day you find out why."
Mark Twain
America and various other countries have the death penalty in place, but their murder/crime rates haven't gone down.
We can't say that the death penalty will act as a deterrent. If someone is caught illegally downloading music and films, they can get an unlimited fine and possibly a prison sentence. Does that stop anyone? No.
MUMFORD