We don’t see inside Jean Valjean’s head when he decides to go to the barricade, so it’s not clear why he goes. But the lead-in to the decision is much like the lead-in to the Champmathieu fiasco: There’s a threat to his way of life, but all he has to do is wait for another man to lose his, and he’ll be safe. From that standpoint, the question of whether he goes to protect Marius or to watch him die isn’t so ambiguous. It’s the same kind of dilemma, and whether he’s decided or not (just as before, it’s Schrödinger’s decision), he can’t let fate relieve him of the responsibility, so he goes.
Valjean has wrapped up way too much of his sense of self-worth in his role as Cosette’s father. That’s what makes the prospect of losing her seem like such a threat. And he’s right: not that it should be a danger, but that it is, because he can’t handle the transition. After nineteen years of prison and seventeen years on the run from the law, Valjean has latched onto supporting Cosette as the one thing he’s good for. And if Marius is doing that, he can’t see any place for himself.
Just like Javert, he dies from inflexibility.
Javert kills himself because he realizes that his world view is wrong, and he can’t see a place for himself in the world as he now understands it.
Valjean sinks into depression and starves himself because his world has changed, and he can’t see a place for himself in the new one.
In the past, Valjean has been incredibly resilient. He adapted to life in prison, out of prison, as a businessman, a fugitive, a father, a gardener, and a retiree. But he’s become stuck here, seeing his life with his adopted daughter as the endgame, not as one more stage in his life, unwilling to let it change until his conscience forces his hand, and then unwilling to embrace it