this is for the confused viewers (spoil alert)
first off: nobody died
the guy just became a "ghost," as in no one sees him.
while in prison, there were stages of metamorphosis, he slowly "leveled up" in a way.
by the end of his practice, he is able to complete hide in the other 180 degree of the human eyes
this ability is what the mystery of the film is about, it's saying that we will never truly see everything in this world because we will always have a blind half that changes constantly while you're not looking.
second: the scale is fixed (in the end)
they're not ghosts, there was a short scene before he comes that she fixes the scale, so it's broken. both of them did not cancel out each other's weight either, it's even mathematically impossible for her to fix it so her weight cancels out his weight - different positive and negative integers do not make 0. the strange scene is suppose to make you second guess yourself about whether they're real or not, when everything is completely real.
great film.