Capital punishment is bad for two reasons: (1) In order to ensure you don't kill an innocent person you need trial after trial after trial. In the long run that means capital punishment costs more than putting someone in prison for life so it's not cost effective. (2) Only a handful of nations have capital punishment. The US and Japan are the only two I would consider honorable civilized nations. I don't particularly like being on the same list with the others (Saudi Arabia, Iran, China, etc).
The morality of killing a murderer doesn't bother me at all. That's not a factor. I don't think the threat of capital punishment prevents criminals from doing there thing but it certainly stops the one criminal from doing it again.
In order to ensure you don't kill an innocent person you need trial after trial after trial
No you don't. You just reserve capital punishment for people who are quite clearly guilty and quite clearly uncontrollable.
Ted Bundy, for example: clearly guilty as hell, impossible to control, killed and raped several more women after escaping from jail twice. There is simply no sane way to deal with someone like that other than to kill them.
Everyone who hast done a crime should be killled that is what happens in my country (in Britain). It is the only way to makes sure that you will not be rapped or shot when you are walking in the streets.