Basically, in RE7, at the beginning of the game, Jack murdered Ethan, but Ethan was infected with the mold, which brought him back to life. This explains why he can recover so quickly and reattach his limbs. Ethan could technically live without his heart, which explains why he was still alive. Once Ethan killed Miranda, the mold started to dissapear, which was killing Ethan, since without it, he's nothing. Knowing he wouldn't survive much longer, he sacrificed himself to destroy the giant creature.