Other users were using an area designed for free combat as a social zone. Perhaps the designers should have created a safe social zone, if that was such a popular thing to have.
He's doing that because he can't attack them. And they can't attack him either. it says:
> the victim has no chance to retaliate
They mean literally. If he was just attacking people who could attack him, in a zone where PvP is enabled, it wouldn't be called griefing.
Also someone in comments below says they have patched this stuff now. Legitimate behaviors don't get patched.
Having said that, however, he did not engage villains in combat, but rather teleport them to a kill zone. That's unethical, for sure. Had he simply engaged villains, I'd see nothing wrong with that. If other players don't want to kill villains, even if that's their job as heroes, that's their problem.