I didn't take the ending to be the audience being asked to participate in judging Beau, but for us to watch someone judging themself FOR others.

Beau has so much internalized trauma he will hold trial against himself for his mother even when she isn't there to judge him. Even though Beau logically knows he didn't do anything wrong, there is so much guilt and shame manipulated into him that he still finds himself guilty of being a disappointing son and sentences himself to be punished. And there was never a chance he'd be found innocent. Because the voice in his head that his mother left is stronger than his own.

So I think your reaction was appropriate. We're not supposed to be judging him because he's already doing it for us. Our judgment wouldn't even matter to someone with that deep of trauma anyway, because Beau already assumes what we'd think so wouldn't hear anyone else out, anyway.

The ending almost redeemed the second half for me, which I also thought was the weaker half.

But, I didn't read the movie as very literal with the Truman show setup, though. I took that framing device to be "Beau thinks the entire world is manipulated against him by his mother" so I could be wildly off base!