But expecting an "evil path" to the main story, including some sort of evil ending, in a game that is set in the Forgotten Realms setting, is highly unrealistic. To put it bluntly, evil is just not going to end up "winning" in a Forgotten Realms game.
I'm sorry, but.. what? Many games set in Forgotten Realms have evil endings, including the original BG duo/trilogy! You could become a literal God of Murder, doesn't get more evil than that, in my book