Well talbot is a potential antagonist, but that can only be happen in Gil's route, and he can't be a villain.
There is a difference between the "antagonist" and a "villain"
Villains are antagonist that is hate by everyone, and he as a villain also hates every character, that is why the word "VILLAIN" is strong calling/naming.
In Talbot's case, everyone hates him, but walter is the only character (maybe) that Talbot want to deal with and one of the MC likes him (somehow) but he's consider as antagonist. (IF I SPOKE SOMETHING WRONG, YOU'RE FREE TO CORRECT)