It's not about offending anyone, it's the principal of celebrating death.
I'm not saying he didn't deserve to die, that he shouldn't have been shot or that him being caught or even killed is not good news, and I certainly do not have any sympathy for him.
All I'm saying is that no matter what form it takes death shouldn't be a means for celebration, Yay we killed the bugger, happy holidays, merry christmas. Break out the beer and the streamers. Hooray woooo!!!
We caught a bad person, we killed him, great the world is a safer place. But it shouldn't be treated as an excuse for a party, that's just trivializing something serious.
See it the other way then. We're not celebrating death. We're celebrating the lives that are saved thanks to his demise. We're celebrating that the people he tried to kill next are safe and do not have to fear for death.
Can we celebrate for people to be safe and not be killed?
If you demand respect or gratitude for your volunteer work, you're doing volunteering wrong.