I think you nailed it. Antiheroes are a power fantasy. If they weren’t relatable, the movie would be boring. I think they also make people feel strong and resilient in comparison: the joker (or whatever) faces some of the same humiliations the viewer faces, but the joker snaps and goes on a rampage while the viewer carries on. The rampage part is the “anti” in antihero. The viewer gets to feel like the hero.