The tearing off of the robes and the casting away of the staff suggest to me that he's basically quitting his job as head prophet, or whatever. Like 'take this crap seriously or I walk' type of thing.
Anyway, bummer on Zandalar sinking, because I really wanted to see the place.
Although... if there's a naga related expansion and Azshara starts raising crap from the ocean floor, that makes Zandalar a candidate. Giant un-sunk troll island full of ruins and ghosts and such...aw yeeaah...
Hmm. Maybe Rastamon will get his shits together eventually and help us in some way in future, after all BURNING LEGION IS COMING OMG. Imagine that! More troll instances in Burning Legion expansion!! "ya mon, kill da naga/demons in our city!"
'Hey man , you killed all my race but i will still always help you because somehow you are always the good guys.'
Maybe technically we are killing only Zul's troops? What if only casual trolls followed Zul and Rastakhan just got rid of dead weight from from his army! Now "true" Rastakhan's zandalari consist only hc elitist raider trolls!