While I'm hoping each playable character, including Tav, has a special ending, I don't want to assume too much about how evil each has to be.

Players probably wouldn't be satisfied picking Gale, SH or Wyll... just to encounter railroading. It's a bit mean to do. D&D players are no doubt among the most critical roleplayers.

Of course, a predetermined ending /may/ be the most logical, but it's limiting. Larian seems too flashy in their style to sacrifice player choice. They're flashy enough that I have to wonder about physical marks of evil (tadpole use).