my theory is that since he knew that one of them had to die, and that he wasnt the one, he went to go sacrifice himself to free morpheus since one of them would live.
although it seems reduntant, cuz he probably wouldve surmised that morpheus was going to die anyways without knowing that one of them had to die, and so would go try to help him anyway knowing he wasnt the one...knowing that one or both of them could very well die anyways.
this also makes sense. she told him (or let him tell himself) he wasnt the one, maybe would be in another lifetime, and he knew that one of them was going to die, so he went to go sacrifice himself to let morpheus live since he thought he wasnt the one...then he died, morpheus lived, and he became the one in "another lifetime."
thats what im going with lol