Yeah, the more I think about it, the stranger her timeline was. It would've made much more sense had Buckbeak's execution taken place earlier.

I don't think it's OOC either, but I do think it's a part of Remus's character that was never fully explored - it comes up again in DH, when he does some pretty questionable things, and it does seem "off" there too because there's no balance or apparent set up for it. Of course, in this scene there's also a Full Moon coming, so maybe some of it was the inner wolf coming out? He might be so caught up in the moment he doesn't even realize it. With Sirius, he was acting like the mad man he'd purported to be until Remus showed up to calm him down and get answers, so I think he's still unhinged at this point.