I was just thinking that there was another aspect that was bugging me about Sam getting mad at Dean for not telling him that Jack was going to die (and allowing it, I guess). It wasn't Dean's secret. It was Jack's. If Cas hadn't broken Jack's confidence, Dean wouldn't even know. If someone told me somebody else's secret, I'd keep it to myself. Well, assumign it wasn't going to hurt anybody else. If Jack's secret was that he was going to sacrifice Sam, then yeah, tell Sam.
It's also Jack's choice. As much as I don't like Jack, I have to say he has the right to make his own decisions.