Nick is getting a hard time about this, but I feel that Joe brought it on himself somewhat!
Unless I'm recalling the events wrong: Joe's stated intention (in the lounge with Nick) was that he'd be forming a fake alliance with Cat and Alan and voting to banish David. Nick looked uncomfortable about the idea of David having to go as collateral, but it made sense in a brutal kind of way (not that he agreed to follow that voting himself). So, the Joe-Nick partnership had an understanding of what would develop at banishment, and Joe went outside to bring Cat and Alan onside against David.
Come the round table, I would have become unsettled had I been Nick, and seen my supposed ally switch from David and vote for Cat! I would have doubted Joe too at that point.