I think this description could definitely be interpreted either way, and that might have been the point. My first instinct when I started the book was that he was innocent. I think the word "grace" is what swayed me more than anything, because the other words like "proudly" and "rigid" aren't very innocent sounding descriptions, but when paired with grace they were conveyed differently.
I think I also took it as him knowing that he is innocent of this crime but was resigned to the fact that for various reasons he would probably be convicted and there was nothing he could do about it, so at least he could remain strong and to be very cliche "keep his dignity/pride intact."