They were still making snide comments about it while we were pulling trash to the Maiden of Grief, and I finally snapped, "He's right. You can spare a second to interrupt the cast, and if you won't do that, the least you can do is avoid killing a mind-controlled player." The abuse rerouted in my direction for a few minutes, but one of the nice things about working here is that after you've been a target of the professional trolling class, their amateur counterparts don't have that much effect.
It leaves me wondering how you deal with unpleasant players without causing a blow-up or turning into a bit of an ass yourself (as I admit I probably was while coming to the defense of the moonkin). We all run into them eventually, and I don't know whether it's better to speak up on behalf of a bullied player who may not want the help, simply drop group or spend the run in silence, not rising to the bait.