I'm curious as to what exactly Shu did in the first half to piss people off.
Was it objecting to a mission where the causality projections were one out of every three Funeral Parlor participants?
If that's it, I find it odd for faulting Shu for having moral objections with military missions likely to result in heavy casualties for both sides. Keep in mind that he has little more than the word of Gai and his Funeral Parlor allies (i.e. which is basically Gai by extension) to go on here, when it comes to the importance of this mission.
Was it him being quickly persuaded by Inori to change his mind?
Well, on the whole, Inori has been nicer to Shu than most/all of the other Funeral Parlor members. And I think he'd rather feel like he was simply going along with her than giving in to Gai and Funeral Parlor as a whole. Shu's relationship with Gai and Funeral Parlor as a whole is more strained than his relationship with Inori specifically, at least in Shu's eyes, I think.
Was it him exchanging blows with Gai?
Well, Gai did say something to him that I think most hot-blooded males in Shu's position would get pissed off by.
In a weird way, I actually have more respect for Shu due to him exchanging blows with Gai. At least the kid isn't a coward, imo.