If you play the Origins as your Avatar, the basic turmoils and conflicts that drive their character arcs are still the same as when they are your companions. Unless you choose to go completely wild, the story gives you reflection-points to consider your character's motivations that are specific to them. If you decide that their story leads them down a dark road, it is still their dark road. For example for Avatar-Gale his personal story is still driven by the conflict between seeking Mystra's forgiveness and seeking power so he might not need it. A brain-grabbing ending in his case should play on these themes to feel fitting. So an evil ending for Wyll should also feel like an especially twisted version of himself, in which he deemed taking control of the brain was the best choice to achieve his goal.
Exactly. And the player is perfectly equiped to make his ending feel fitting. For example, Wyll was never in control of his life since Mizora/Tiamat happened. He did best with the cards he had, but it wasn't enough, not even close. He got used and killed Karlach. He failed to protect the Last Light In, even with warlock powers, a lot of people died to the curse, as he thinks, on his behalf. Then he immediately got abused by Mizora once more, pulled into the resque mission against his will. He failed to save Yenna from Orin, he failed to rescue his father, Gortash controls him. He had enough of being weak and the promise of power seems real, he goes for the Astral-Touched Tadpole. His circuits rewire, he becomes more calculating, more pragmatic. With the newfound powers, he sues for peace with Gortash, eradicates Orin. Drunk on success he schemes to sabotage his truce with Gortash and overthrow him as well, sides with the Ironhand terrorists. By that point, Wyll is accustomed with the concept of collateral damage, he is not a fairy tale hero, nothing comes without a price. Sides with the Ironhand terrorists, destroys the foundry, let's them kill the gondians. By this point Wyll can be considered a neutral evil character, who fights for control over his own life and power to take it. The further developments could lead to control over others as easily. This is just a single example.
