I'm also against capital punishment, an entirely different beast. It kills innocent people & giving the state the power to murder doesn't result in justice overall and isn't civilised.
But when awful people die, who left terrible marks on the world (so many to choose from), people are well within their right to take solace, joy or whatever in it. Doesn't have to be your own approach, but all 'don't celebrate death' does is show privilege and disregard for suffering, I think.