The way i look at it, allying with goblins doesn't even make much sense for an evil char. What does make sense is removing them (or rather the 3 leaders)
Considering that:
a. You embarked on this journey to remove the parasite. That's ..the name of the main quest. It's what you want. It's what your companions want.. if their dialogue is to be believed. Now, the only real 'promise' of a cure comes from Halsin, who was kidnapped by goblins. And to get Halsin, you need the goblins gone. So as an evil character, a pragmatic one, who wants the parasite gone..why would you choose the goblins over Halsin?
b. If the goblin leaders find out that you want the tadpole removed or that YOU killed the mind flayer, they will attack (as will the 2 humans you encounter early on near the grove, if you tell them you were on the ship). They are a threat to you, so why would you want to let them live?
c. You learn that they have tadpoles of their own. If you know what power these tadpoles can offer, wouldn't you technically as an evil char want your competition removed?
d. The goblin leaders offer you nothing, as far as i've noticed. No promise of a cure, no promise of riches or power. Maybe i somehow missed this conversation, but in my game the drow (Minthara?) just went 'oh another true soul, come, sister, let us attack the grove!'. Ya ok, but why? Why would i want to be their underling, and potentially be paid in potatoes, if removing them is so much easier?
If there was a way to manipulate the cult leaders, if you could take over this branch & end up with an army of goblins to command AND SO make your path forward basically easier, i'd understand the appeal. But as it is rn, killing them is the only logical option.
