Some chapters start with alif-lam-mim. As they say, nobody but Allah knows their meaning.
I joked that perhaps they mean "take it with a grain of salt". A guy nearby, a devout Muslim, went furious. He said something along the lines of "if we had not been colleagues I would have to [destroy] you" (we were both grad students at the time). He did not say "destroy" but some other word to that effect, I am not sure which one, maybe "kill". He knew I was joking but he was deeply offended.
I think people have a right to be offended if they choose so. And they have a right to joke if they choose so.
They have a right to defend themselves when they are threatened. I told him I would defend myself.
Maybe he felt threatened by the joke? Could be. His problem.