Why do we have to keep dangerous terrorist alive?

I read in the LA Times today that a man who pleaded guilty to trying to destroy the Toronto stock exchange (with a bomb three times as powerful as the one in Oklahoma City) is getting only like 14 years in prison. This coming after the release of the Lockerbie man makes me mad. Why do we all as NATO countries let this happen. If you wish to commit m murder, shouldn't you be executed. Same thing at Gitmo. I know there are a lot of people that didn't do anything wrong and are still kept there but what about the ones that succeeded in killing Americans (like this one man who we released who killed many people in a grenade attack). Why do we pay taxes to keep these men alive?