Well, first of all, we do need to go through the trial. I know they've caught him, I know the evidence we've seen in the media seems overwhelming, I know it's been reported that his flat was booby-trapped, etc, but it is important for due process to take place. Niceties, perhaps, but important ones.
However, if we assume he is found guilty, I am personally still against the death penalty. It's hard to maintain a deterrence argument, when your future perpetrator is highly likely to be deranged - and as a society, I still like to feel that we are better than that.
I do understand the desire to put people to death, if they themselves commit murder, but to preserve our own aspirations at civilization, and to prevent even one innocent person being put to death, I would still cast my vote not to have a death penalty.