I would argue Trevor DOES become the final Antagonist if you choose Option A "Something Sensible".
That's what makes it quite good in my view, you can twist the story around depending on what you prefer. If you pick C the big bad is Devon, and to a lesser extent, Haines, Cheng and Stretch. If you pick B, it's Michael.
I hate these responses, what I mean for an antagonist is someone who is on the opposition of the protag throughout the story, who doesn't appear alot and who is an NPC, like Dimitri or Tenpenny
Not some half assed antagonist depending on 'how you look at it'
And here I thought you wanted to have an intelligent discussion. Personally I prefer the "depending on how you look at it" antagonists. I like that grey area, where ultimately you feel like you did the right thing, but still can't shake the bitter taste from your mouth. There are not cut and dry heroes and villains in real life, people have conflicting motivations.
I think Trevor would have made an excellent antagonist if they hadn't made him go out like a bitch in ending A. I would have expected that crazy motherf*cker to come at me with a minigun, not wallow in a pool of gasoline crying like a little girl with a skinned knee. Maybe after you stop his minigun rampage.
At any rate I would have wanted to use him for the entirity of the story, because he is also a lot of fun.