Hught, I've often wondered the same thing. As a Christian, how can I say, an eye for an eye? So if I kill you, that means someone needs to kill me, right? So what about the person who kills me? Didn't they kill too? Not saying what's right or wrong, only wanted to interject that I've often wondered that very same thing.
Actually, I have been told that due to translation that particular verse has been changed, at least in its meaning. Told that the original intent was "no more than an eye for an eye". I think regarding your killer becoming a killer is also a punishment on the individual that has to carry out the sentence.