God gave life and only god can take it away. In murdering a murderer you are playing at being god and had no more right to do so than them in the first place. Even if he does deserve death no human is qualified to make that decision. We are on gods earth at gods grace and mercy. He said thou shall not kill. Full stop. He didnt add "unless they already killed someone else then that makes it ok".
Im slightly hypocritical in my arguement though as I am glad to see some people still put to death in USA because of their crime. But from a moral/ethical and christian
perspective it is wrong no matter how you dress justice up.
Sometimes I think "good they got what they deserve" but despite this, I am glad its not practiced in England because its just too messier an issue. And as someone said the possibility they might be innocent is too horrible to conceive. Imagine being on deathrow for something everyone falses believes you did. Yikes.