I think the core problem is two specific points in our argument. I do not believe it would be a win to drag the PC through the underdark, UNLESS the DM changes his mind on the PC limit. I also do not believe something truly necessary, to the point of the death of the entire human race or even further, the death of the planet, can be Evil. Unsavory, most definitely. A guilty burr in one's mind for the rest of one's life? Possibly, depending on the underlying psychology of the character. I think what is far more selfish, if you are on a quest to save the world, would be keeping your old friend alive, even when they seriously jeopardize the saving of the world. Unless of course you planned to ditch them in the middle of a fight, as soon as they start making things more difficult.