What is this discussion even? Wyll is only destined for an evil ending if you play as him and lead his character there. The game provides a wide plethora of interesring situations and meaningful dialogue options, a lot of which can be interpreted as plot turning points for any desired transition. If you fail to develop a character properly in order to be satisfied with the given state of affairs in the end, thats on you... we are talking a role-playing game here, not a scripted dialogue simulator where every story bit and decision is interpreted for you, in great detail. You need to do some RP between the lines to make the things feel right and organic. If you do some actual role play you'll never get into a situation when a goody-to-shoes suddenly enslaved the world or any other absurd situation.
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.
