If they weren't laughing, they'd be crying. In order to overcome a tragedy, we have to stop collectively mourning it at some point, become stronger than the tragedy. However, mocking/joking about those that suffered at the hands is still quite inappropriate; it's like prodding at a dog to make it bark, abusive.
But being offended by jokes about the people who committed the crimes? Or the people who have open bleeding hearts for something when it, quite literally, didn't impact them? Stuff like that, there's no point in being offended by the jokes. I'm all for standing up/sending well wishes for the victims and taking down those who coordinated the killers, but unless a family member of yours died in the event, don't sit there weeping about it.
There's always a key in moderation.