To be fair, Wyll is also the one who can actually oppose the main character and defend the grove against them. Problem with Karlach and the pact lies not within Wyll's lack of character, but in the overwhelming difficulty of choice. It is not the choice between 'evil path' and 'good path'.
You believe Karlach and spare her, you potentially get turned into a lemure and the Coast loses its sworn protector - many more families die at the hand of beasts and men.
You believe your source and kill Karlach, you gamble on saving the Sword Coast from the potentially dangerous infernal.
You agree to break the pact and forsake your father. That means the Gate loses its great general who contributed greatly to pursuing ideals that are embedded in Wyll's heart and mind. That ALSO means Wyll is now free from hurting the realms on the devil's command, there won't be new Karlach he needs to kill, there won't be time spent differently than helping the people.
Wyll's story is a balancing act, there is no obvious choice here, thats why I think its important to propose some hidden system aka 'nightsong points' for the change to be helpful, not harmful.
