The role of humour, if you listen to any of the comedians talk about why they do what they do, is to point out things in our society that people don't want to talk about. This is partly why humour isn't funny when you make the same jokes over and over, and it's also why comedians love to jump on recent events and news.
Things are "funny" because they're unexpected; it's a positive type of shock. Some people believe this kind of reaction is "offensive", but I think it's important to open up the floor and make us feel comfortable with talking about the issues. People get so couped up and tight-lipped about tough areas; why can't we have a laugh, relax a little bit and then come back and be serious on those issues?
Now, yeah, it might be wrong to directly laugh in the face of those who've lost someone dear and near, but in the majority of cases you're probably not involved at all, so feel free to lighten up a bit. That's exactly what those horrible people don't want; their goal is to destabilise our culture and put us into a state of fear, which would allow them to easily convince people to join their side as a matter of survival.
Besides; in laughing at jokes surrounding these kinds of terrible attacks, it in a sense removes the emotional charge they're supposed to generate. People aren't going to commit horrible crimes if there's nothing to gain from it, and what they want (when it's not some form of tangible profit) is either a reputation or a reaction.
