You can't blame him for being "chipper." He already doesn't like them and now they're about to get in trouble for essentially hijacking his ship and putting him through hell.
Well, duh
In his head, he was probably telling himself he was sticking it to Locutus....
I thought it was perfectly understandable. Picard and Riker hijacked his ship and almost got it destroyed and his crew killed several times over the course of, like, four days.
I'd be pissed if I were him.
Oh, I don't blame Shaw at all. On the contrary, I thought those scenes were great.