I am really confused on the topic, because for me there are 2 main things to consider:
1) ending someone's life of course, which is murder, if you murder a murderer that makes you one too

2) on the other hand, this man commited something really horrible, what is the alternative to executing him? life sentence, or at least 20 years in jail. in those years he will cost the public money for food, guards, a room, health maintenance(what if he gets sick

) possible damage cost (in case he escapes or kills someone in jail) electricity etc... and in some countries internet, libraries and other stuff. for 20 years, that could amount to half a million dollars for every single prisonner, should the public be forced to feed a criminal?
and that is why I am confused :P