Light Yagami, the protagonist of the anime series "Death Note," experiences a significant deterioration in his mental health as a result of his actions. His initial belief in his self-righteous mission to rid the world of criminals through the use of the Death Note leads to an obsession with power and control. This obsession, coupled with the moral weight of his decisions and the constant fear of being caught, gradually consumes him and drives him to increasingly desperate measures. Ultimately, Light's descent into darkness and megalomania highlights the devastating impact his actions have on his mental well-being.