Right, wasn't 100% on remembering that part. It would have to be some well done smoke and mirrors at this point involving a god or something manipulating the Mind Flayers for their own goals though I can't see it being the case. More than likely this means the main story path is getting rid of the parasite and potentially stopping an apocalyptic invasion. If that's the case maybe they're aiming for evil and good choices to be less linear paths and more divided into the individual quests? Taking that into account I imagine there was never an idea for characters to stick with the cult and their intention was more use it to figure out what is going on and how to get rid of the tadpole. Which is certainly the darker path, killing the teiflings and druids to accomplish it, but its definitely an out of the way choice with little incentive or reasoning. I could also speculate a whole bunch of ideas on where the story goes and what could be the "evil" path but from what we've been given so far, and as has been covered, you're really not incentivized to follow it. Hell, everything you learn through the story is that it's not good for your health, don't use the tadpole and get that sucker out quickly, stasis or no stasis. Safer to work with any amenable people to get rid of the tadpole and more than likely stop any Mind Flayer plot.
On the whole If their evil path is just a whole lot of slaughter the innocence for the lols I imagine I wont be choosing it, though I have done a run through of it. That's something I'd do in Fable because I was bored or wanted to purchase some more houses. Another part to think about is the fact that so far most of the evil characters are enslaved by the tadpoles and don't even know it which makes it more a priority to either kill them or get the tadpole out of their heads rather than aiding them in accomplishing the Absolutes plans. The more I think about this the wilder, and not in a good way, the evil path becomes.