[Mortui]

The entire maps of Hub areas (Valstiir, Hadrian, Sullivan's Port) require a DM to PvP in unless you're using fists.
If you intend to kill a monster PC in Sullivan's Port, you still need a DM, because blue NPCs will react, even if they're monsters!

Additionally, if you're going to PvP a monster PC, you need to be on hostile before attacking, just like with non-monstrous PCs.
You should give consideration to monster PCs if you intend to kill them, give them a chance to flee or beg for their lives and generally don't be a murderhobo.