Im #3 for the most part, though there were a few times since the CATA laucnh where allies kept grabbing mobs / quest things right in front of me and id get fed up and unload.

If I see an ally attack a horde though I will try to help them. This is potentially crappy because it could've been the horde player picking the fight, but theres no way for me to know all the details so in situations like that i just side with my faction.

Usually though, just self defense. I actually got ganked last night by 4 level 85s (my dudes are 82). I killed one of them, but i lost the battle due to poor target selection on my part ( i was attacking this pally when I shouldve been on the priest). Otherwise I think I wouldve prevailed =P I dont pvp much so i tend to panic a bit in those situations lol.