Just have Minthara explicitly promise you power from the Absolute if you give up the location of the druid grove and/or participate in the raid. You'd be able to join the cult with their recommendation. That's a perfectly valid (non-murder-happy) "evil" motivation: power at the expense of others. This could even work for more neutral or even good characters: you're trying to infiltrate this cult in order to learn more about the tadpole in your head and/or to eventually take down the cult: sorry tieflings&druids you're just collateral damage/it's for the Greater Good.
You can do all that even now, just presuming that ...
Is that sentence really so important? :-/ Do we really need DM to hold our hands and give us huge neon sign on every single crossroad? :-/
That seems like a pretty foolish assumption to make. "Yes evil cult, I totally believe, based off my interactions with Minthara and Dror (who both are arrogant, belittle your character, and give your character commands) that by helping you you'll give me a nice position in the cult and share cult secrets with me."
Regardless of whether such a line
needs to be in there, it would make the game better if you could roleplay bargaining with Minthara for your information on the druid grove location. And as @Gray Ghost says, it makes sense that (smart) characters would want to know more about the cult before joining.
