tmnt spoilers

Leo’s forced second mutation caused him to become like the one thing he hated the most…

That leaves me to assume, that the Shredder never left the forefront of his mind. Even after he defeated him and showed his family Shredder’s severed head ((in true samurai fashion)). His arch nemesis, the man who murdered  his father has probably always and will always haunt his nightmares..

He had tears in his eyes when he tried to push his brothers out of harm’s way. :,(