My only moral limit is causing someone's death.
When I first started trolling I never told anyone to "kill himself" in fear of him actually acting upon it, but I've grown to realize that nobody online takes your words seriously anyway.
Of course, there are some mentally unstable people who can commit suicide upon reading words on a screen, but I tend to recognize those and avoid urging them. Most of the time they're not even lulzy targets, more like extremely sad/depressed people not worth the shitposting.
On the other hand I think that if someone's can't handle a few written words to the point of killing himself then he was not fit for the Internet to begin with, and that should not be my problem. I believe only he is at fault for browsing the net in an unrestricted manner knowing it's potentially dangerous.
One should always know what he's facing and adjust himself for it, or otherwise just quit.