There's no evidence that capital punishment deters anything. None, zip, zero. Study after study finds no correlation or even a lowering of crime in those places that do have it.
If that's your argument, think of another, cause it holds no water.
I'm not going to shed one tear over this guy. There are some people who deserve it.
I however am against capital punishment because I know that our system is not perfect -- I know that sometimes innocent people get convicted and guilty people go free. And I don't think we should have a 100% irreversible punishment in a system that is not 100% perfect.
And no, I wouldn't make exceptions. ("But this guy confessed and is really terrible!") Once you start making exceptions, it just opens it up and soon there are more exceptions and more and then we find out we've executed someone innocent.
Lock him up forever, like Charles Manson and many other terrible people who will die in jail and never see freedom again.