I'm not sure I'd classify the extra brain slugs as an evil choice as much as a stupid one. "Okay, so I'll be even more likely to die or die even faster, but I'll be more powerful! Bwahahahah!" Not that stupidity and evil are mutually exclusive of course.
The only way I can see it not being a stupid choice would be A) if we have some VERY good evidence that shows that no matter what we do, we're immune to the tadpole effects or B) we have nothing to lose, are going to die regardless, and might as well be powerful and do what we can for the little time we have left. If it's played as just another valid choice, you're right, it's not simply evil, it's just dumb, which is what I thought Larian was trying to go against, having Evil NOT be just evil/stupid.
