The reason to take the evil path, should be a role-playing one.. But I do agree as far as that the evil path needs work. As it exists currently, it is too punishing on the player, since you lose access to all the merchants in the area, so you can't buy or sell stuff anymore. if you want to roleplay a cleric of Bhaal, who uses murder as a form of piety, and decide to kill the companions instead of recruit them, you cut yourself out of a tremendous amount of early game xp, and gimp yourself essentially.
That is why the evil path sucks in my opinion. Roleplay wise, its fine. game-play wise, it's not worth what you sacrifice. And what you sacrifice, makes the game less fun.
Is there another role to play than "I want to play an evil character" to side with the evil forces ?
I really tried, but I cannot think about ANY other reasons to side with Minthara at this point of the story.
You just heard a bit about the absolute and heared a bit (mostly in the goblins camp) about a cult with goblins and "true souls" leading them... you just don't know at all why you're not turning into a mindflayer, and the only thing Minthara suggest is that you become her minion.
