From what it seems...they're not evil. They're just working towards their own purpose, as they always have.

Up until this point, your goals and those of the tribe have gone together. Now, that's no longer the case.

It's more interesting when you don't think of it as a binary good/evil, but rather different factions with their own goals that sometimes run counter to your own...and in those cases, they have to be stopped.

They haven't turned evil. They're doing the same thing they've always done, which is look after the troll race and clean up the worst of their messes.

It's just that this time, they're looking after their race in the face of the Cataclysm by uniting all the tribes and the two empires. They're still good guys. Just good guys in a way that doesn't necessarily agree with Horde and Alliance agendas.

The Horde and the Alliance frown on this since a united and organised Zandalari empire would probably be more powerful than both factions put together and have a troll orientated agenda that would probably not bode well for everyone else.

Vol'jin also frowns on this because
a)Those other tribes are dicks to the Darkspear and would not enjoy renewing his peoples position as bitches of the troll race.
b)He's part of the Horde.

The Zandalari want trolls as a race to survive. Before, they agreed with the player's desire to defeat Hakkar the Bloodlord because he required massive human (or, in this case, troll) sacrifices. The Zandalari want trolls to be safe, and if anyone wants to help them, then they have no problem with helping them in return. Mess with 'em, though, and they'll slice off your head and shrink it.

I'm actually excited with this, because (as far as I can tell) this is the first time we get to fight "good" guys. Usually we fight obviously evil people (excepting PvP), and it should be fun to fight against a faction that we used to ally ourselves which, but whose goals no longer coincide with our own.

Big fun! :)

It would seem that being good only was a setback.

Having us fight "good" guys with conflicting agendas seems very interesting. However, I'm afraid that as the story progresses, it'll turn out to be due to just another Old God/demon/otherwise evil influence. I hope Blizzard proves me wrong on this.