You're right. I did conflate the everyday meaning of 'antagonist' with that of Dramatica's. This happened whenever I brought in the morality aspect of 'the bad guy' and especially when I said 'if you ask anyone they will say...'.
Beyond that, though, I'm not confusing the everyday terms with the Dramatica terms in my post. Every other use of 'protagonist' or 'antagonist' refers to their dramatic functions and elements per Dramatica theory.
The protagonist pursues the OS goal and gets others to consider its importance.
The antagonist avoids the OS goal being achieved and gets others to reconsider its importance.
Applying this to Infinity War:
The Avengers and allies pursue the goal of stopping Thanos from collecting the Infinity Stones. They get people to consider how important it is to stop him.
Thanos works to avoid (prevent) the goal of the Avengers and allies stopping him from collecting the Stones. He gets people to reconsider whether they can or should stop him.
These conflicting purposes provide the dramatic tension for the Overall Story.
In regard to
You can easily explain almost any villain using these 'protagonist' words. Try it with a Bond villain. They're all pursuing a clear agenda, certain in their morality, proactive in working to achieve their goals, seeking a certain effect, and confident in their knowledge of what they can/will do with their ultimate weapon or whatever. So, used this way, because these terms can be used to explain so broadly, they end up explaining nothing.
Remember the Protagonist is for the Overall Story goal. The Antagonist is against it. So really, it depends on what you think the story is about and who's viewpoint you think the writers wanted us to take.
If you had to pick one, do you think the story is more about:
A team of heroes trying to stop a villain from doing something bad?
An antihero trying to get past a bunch of obstacles to do something morally right?
If the former, the Overall Story Outcome is Failure, in Dramatica terms.
If the latter, the Overall Story Outcome is Success, in Dramatica terms.
Further, if the Avengers failed to stop Thanos, do you think the individual Avengers characters resolve their personal issues? If not, you've got a Dramatica Tragedy.
If you feel Thanos is the Protagonist then he Succeeds in achieving the goal. And his smile at the end shows he's content with this outcome---his angst is resolved. This gives you a Dramatica Triumph.
How do you think the writers wanted you to feel about the story's ending? Elated or despondent? Dramatica, when used for analysis, is attempting to determine author's intent. But regardless of author's intent, how did you actually feel at the end of the movie?
I think, as others have mentioned, we do at times sympathize with and understand Thanos. I understand those who root for him. I love this tragic ending because it's uncommon and compelling.
Making an effective villain, one not shallow, requires giving the audience some insight into the mind of the Antagonist (a literal serial killer in this case) and even understanding their skewed perspective and how their abhorrent behaviors logically follow from this perspective. This doesn't make these antagonists into protagonists. It makes them deep, realistic and effective antagonists.
I hope this helps clarify.