My two cents' worth:
Concerning Jack's behavior after he and Ennis had the fight at the last meeting, I don't think he had a "death wish"; as ww said, he is a life force. However, in a temporarily depressed state of mind, Jack could have gotten a little careless. Partly just being tired, feeling less guilty himself, etc.... but partly, perhaps subconsciously, something of this impulse may have crossed Jack's mind: "I don't really care if I get beat up or killed. Without Ennis, what does it matter." Like I said, not that he really meant that, but at the time, it might have been a conscious or unconscious impulse.
Some of you have heard this before. In fact, I'll quote from a thread I started a while back.
"Near the end of the movie [trailer], the three assailants are mentioned. One, called "Grease Monkey," is played by Christian Fraser. Then, when the stunt doubles are listed, one of Ennis's doubles is Christian Fraser."
My theory on this is that Ang Lee wanted to imply one of two things. Either
a) Ennis was, in some way, responsible for Jack's death. By pushing Jack away, he pushed Jack into the careless behavior that got him killed. I am so sorry to type that. :\'(
b) Ennis feels guilty about Jack's death, and envisions himself/his alter ego present at Jack's murder. That is very sad also. :\'(