Devilman Crybaby

Akira's best friend tells him that ancient demons have returned to take back the world from humans and suggests that he unite with a demon. Akira successfully does that, transforming into Devilman, a being with a demon's powers but who retains the soul of a human. This sends Akira into a brutal, degenerate war against evil. With fear of demons gripping the world, humanity devolves into paranoia and violence and Akira sets out to save other demon-human hybrids like himself.