Devilman. Omnibus Edition by Go Nagai

A tormented teenager merges with a demon to gain the power to fight ancient demons reborn into the modern world, but as he defends humanity he discovers that the line between human and monster is fragile; manipulated loyalties, rising paranoia, and brutal clashes escalate into societal breakdown and apocalypse, forcing heartbreaking choices that reveal the tragic costs of violence, identity, and the will to survive.