I don't like to celebrate the death of anyone. And, to be honest, I don't think most of the celebration is that he's dead. People would be celebrating the same if he'd just been captured. (Admittedly, a lot of them would be building gory fantasies about how he should die. These are the same folks who are now building these fantasies about how he should have died. Their disorders are their own problems.)
Before death, there's always the possibility that someone can change their mind. Repentance is a powerful thing, and there's always hope. (Yes, I'm an optimist. Sue me.)
But, he's dead, and it's probably better that way. Yes, now he's a martyr, but as a prisoner he'd have been a bargaining chip. One can threaten to kill people if a prisoner isn't released, but it's hard to do that with a martyr. The terrorists can just kill people in revenge, but it's not like they weren't trying to do that anyway while he was alive.
So, he's dead. I won't rejoice in his death. But I'm not about to mourn him.