Ending A is basically the "antagonist" ending" to your "antagonist" point of view. Again, if you choose to see him as such.
You don't like to pay attention to detail when reading posts I'm guessing ? Me and Raavi have both stated that we are not talking about being antagonist for a very short duration of the story, like 2 minutes at the end.
You probably won't be the last person on here to come wading into the thread stating the obvious which does not even apply to the discussion.
Likewise, on that paying attention to detail part. You seem to love aggressively responding to people. See? I can make judgements and assumptions based on text responses, too!
Because I clearly stated that Ending A is the antagonist ending not the entirety of the antagonist story. Notice how I also emphasized "point of view". Meaning, you COULD, again, if you CHOSE to, view him as an antagonist as the story progresses. Considering his actions that constantly disrupt the (in comparison) "calm" flow of Michael and Franklin's crime activities.
In example, Michael wouldn't have had to hide out of LS for a time if Trevor didn't go off the rails and kidnap Madrazo's wife. He HURT Michael in this way. You could easily view this as an antagonistic action and add that action to Trevor's (hypothetical) "Antagonist counter".
This is just the example or archetype for a behavior Trevor likes to repeat throughout the story. Basically, getting Michael and Franklin into sh*t that they didn't need to be a part of in the first place. Not to mention being a loose cannon (as stated) as it is.
All ending A does is provide appropriate closure based on how you see it.
That's why people who love all 3 protagonists (or just like Trevor) view ending A as horrible. While people who dislike/hate Trevor view A as an adequate closure to their opinion on him throughout the story. (Generalizing unintended. Opinions vary, obviously.)