I think that even Evil PC will want to resist the tadpole. Take Lae'zel or Shadowheart, for example. A Githyanki warrior and a Cleric of Shar, both evil and both want to be rid of the tadpole for their own respective reasons. The Githyanki hate the Mindflayers and yet they still want to conquer the multiverse. Shar has been one of the evilest gods in the Forgotten Realms, certainly one of the oldest, and she wants people to worship her. The Mindflayers don't revere any gods, so no worship.
Don't forget that traditionally evil rarely sees itself as being evil. Devils, for instance, see themselves as the great defenders of existence, keeping the neverending hordes of demons from the Abyss at bay so that we all can live.
The tadpole isn't just a threat to our own lives, it is a threat to one's very identity and personhood. Not only are you killed by this thing, but your own body is also twisted and mutilated into an aberration that will continue to plague the realms, everything that you were, everything that you could be, gone forever and all that is left is a brain-eating monster.
