But ignoring which encounters/paths you actually choose, the game, as it currently plays, actively discourages you from using the tadpole through the camp dreams/cutscenes. That's OK, as most people wil want to play the "good" path; just makes it less obvious how to be "evil" to help the EA process.
I don't think using the tadpole should be presented as evil. Maybe eventually the tadpole takes control and makes you evil, but even if that's the case, the path getting there should be a little more ambiguous. Especially because an evil character would be even more likely to be against the idea of being consumed and transformed into a mind-flayer as this would not be of any personal benefit. Meanwhile, a good character might justify personally sacrificing themselves in order to achieve a noble goal. I think the tadpole offers interesting choices in balancing the personal cost of allowing the tadpole to grow in strength vs the benefits of the power it provides.
Evil choices should be about personal gain at others' expense. For example, helping Minthara and the goblins slaughter the druids because the goblins seem to be better equipped to help cure you of the tadpole.
I also think it'd be interesting to have ethical backfire. For example:
Good version: you find an innocent person accused of murder, but you're forced to lie to protect them and the lie has unexpected negative consequences you have to deal with and you're eventually forced into a decision to confess that you've lied (condemning the innocent to the death penalty or something) or commit further crimes (perhaps even murder) to continue covering it up.
Evil version: you ally yourself with an evil character (eg the goblins?) for personal gain, but it turns out that you're just a patsy and you get robbed (involuntary charity lol). Maybe you try to ally yourself with Minthara as an evil decision but it turns out Minthara can't trust you because you're just too evil for her (she's not a fan of being backstabbed). Or something else goes wrong for you and you get caught and punished somehow.
The harder you try to commit to a particular alignment, the harder the game should challenge and punish you for it.