Well they probably should've given him a fair trial in the first place but i see your point. I still don't think it would've been a good idea to stay around though. Also i just remembered that the Imperials were torturing Stormcloaks in their dungeon. I don't think the Imperials are in any way playing nice.
Believe me, I'm no fan of the Imperials either, but for the life of me I can't get behind Ulfric. He's a great public speaker, and very charismatic, but I can't help but distrust him. Part of it is his betrayal of the Greybeards. While I don't think it was wrong for him to back out of becoming one of them to fight the Dominion, I think he betrays them through the misuse of his Thu'um. If he didn't want to be a Greybeard, he could have simply stopped shouting, but instead he uses it for war. I know it was originally a weapon, but the people who taught it to him taught it to him so that he may use it a certain way, and his betrayal of that idea is akin, in my eyes at least, to someone studying to be a surgeon so they can slowly torture people without killing them. It's a perversion of their teachings.
The thing is, I could be okay with him using it against the Dominion because he wasn't doing it for himself, but rather for Skyrim. However, I view his use of the Thu'um against his High-King to be completely vested in his own selfish ambitions. He could have beaten Torygg in direct combat anyway. The use of the Thu'um was to make
him look better, and to instill fear in those who oppose him. That isn't for Skyrim. It's only for Ulfric.
Getting back on topic though, I think Tuliius is sadly under developed. He's overall pretty bland and there's not much of a character arc for him. I wish they had fleshed him out more, because they truly could have made him another good figure for the game.