@hypothon I think it was mentioned in the earlier chapter that Ares originally never cared about politics and doesn't care who's the king since he's always in the frontline, so unless he was thoroughly deceived that the underaged king was an 'evil tyrant', unless got a clear motive, it's highly unlikely that he'd realize the green tea princess' plan in the original story and give his support to take down the king.
It's more likely for him to support the green tea princess because he was blinded by love though. Maybe they got into a one night stand in the original story as you said. Face it, this ML is always controlled by his bottom half. Lol.