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.
Ya'll playing neutral characters. Your letting reward decide what to do, not swayed either way by good or evil. So stop kidding yourself.
A: Halsin is kidnapped by goblins, meaning he may not be very powerful. This combined with fact you, yourself can kill the goblins would lead your character to believe he's not all that. So he's not sure thing. After all if he was this powerful why'd he let himself get caught in the first place? As an evil character why would you help someone to weak to defend themselves against goblins, you need someone strong enough powerful enough to remove this thing from your head, not be captured by goblins. If he is that powerful, then that means the goblins are even stronger, or their leaders are. So they're more likely to be able to help you. Plus fact he is a goody two shoes, who will help other goody two shoes, including heros that are annoying, and a threat. Plus can't be manipulated, or controlled do to their boring moral code. Unless you get stronger! Since your not changing yet.... and can use new ability.. hmmm.
B: You don't know this when you go to ask them for help, why are you telling them this, better to find out their plans first. (again in character) You are evil everyone is either 1. fodder 2. potential slave 3. useful ally to be later discarded. 4. A threat to your power 5. able to help you grow more powerful then be discarded. in the end as an evil person when they are no longer a use they're discarded. Dscarded more then likely killed, tortured, maimed, most of it for fun, first one so they don't try to usurp your power.
C. Why haven't they changed into mindflayers? Why haven't you showed any symptoms? Why can you use the tadpole, to do things and not change? (A new source of power for you to exploit? These people know lets use them, help them, get in good with them, learn their ways and then discard them.)
D. They offer you a way to utilize a new power per C. by fully understanding what is going on. Your so bent on getting rid of this tadpole (a thing a good person would want to do, or one who is fanatically against mind flayers.) why if it's not turning you into a mindflayer, then it's a source of power. A source these leaders seem to understand, so you can use them to find out about it. Then of course they have to go once they are no longer useful.
Your supposed to be evil, why are you looking at short term all the time? Why do you want the enemy to be stupid evil? Does a wizard/necromancer want to turn into a Lich for the extra power simply find what they need. No they have to work for it. So by fact it's not an immediate reward why would they bother? They're evil it will grant them power, but they have to work for it. It's a long road, which they have to follow to it's end. A thing your denying your characters because your interested in right now, give me, give me, give me. I must have things now, it must all be mine, right now.
stupid evil = allowing an unknow to waltz into your camp, give them rewards before they prove themselves, and their uses to you. Allow them free run of the camp, without knowing their intentions? Allowing them to live when they are going against what you want. (telling them you killed the mindflayer, and want tadpole gone.)
