You don't have to follow the orders of an unofficial faction member. It's down to roleplay and the situation. Interrupting your RP just to kill the rogue is a nice irony if he really did accuse you of being a norp pvper.
All around, it's a dick move but it's not really sanctionable.
As for your question, you're not forced to follow the orders of the official faction either, it's however very much encouraged. Official factions are supposed to have some standards and you can usually work out something with them.
