As his mental strain becomes more severe over the course of his investigation into Rachel's disappearance, his search takes him outside Ashecliffe's walls to the seaside cliffs and Shutter Island's ominous lighthouse, just off the coast.

Source: https://collider.com/shutter-island-ending-explained/

He joined the League of Villains as a last resort, unable to find professional or personal support and guidance to deal with his Quirk and the mental strain it inflicts on him.

Source: https://www.cbr.com/mha-tragedy-antagonists-could-have-been-heroes/