Devils in Chainsaw Man originate from Hell, arriving in the human world to feed off humanity's fears. The more people fear their name, the more power a devil accumulates. This allows devils like the Gun Devil to become insanely powerful by committing atrocities and invoking fear in the masses.
Devil hunters in the series face off against these devils and try to exterminate them. Denji, our protagonist, becomes a devil hunter, going up against some of the strongest devils in the series. However, exterminating devils is no easy task as killing them doesn't always mean getting rid of them.
Disclaimer: This article contains major spoilers from the Chainsaw Man series.
The mystery behind a devil's death in Chainsaw Man
Devils are effectively immortal, with ones being killed in real life reappearing in Hell and ones dying in Hell reappearing in the human world. As long as the name of a devil is feared, it keeps on reincarnating. However, a devil cannot recall any memories from its previous life after its reincarnation.
The only thing that can kill a devil is our protagonist, Denji. He is a hybrid with the Chainsaw Devil, Pochita, making him one of the strongest characters in the series. Chainsaw Man has the unique ability to erase any devil and its name from existence by killing and devouring them. The fear represented by the names of these devils is also erased from existence, with the human world having no knowledge of their existence. Things like Nazis and nuclear weapons were erased from existence by the Chainsaw Devil, according to Maikma, after he killed and devoured the devils associated with these names.
Devils that get killed by the Chainsaw Man can't reincarnate anymore, not in hell nor in the human world. This power makes him a threat to all other devils, earning him the moniker of "The Devil that Devils fear the most" and "The Hero of Hell."
Chainsaw Devil can be called public enemy number one for the devils and a hero for humanity. Makima even stated that the fear devils have for him is the reason behind his overwhelming strength. Thus, the power of devil erasure falls perfectly in line with his habit of killing only devils.
In the past, Pochtia went up against the Four Horsemen (Control, Famine, War, Death) showing how strong he really is. In the Control Devil arc, he even managed to kill Control Devil, Makima, even after being weakened by her. The strength Chainsaw Devil possesses puts him just below Primordial Devils, like the Darkness Devil, who draws power from humanity's fear in the darkness.
Devils roam in abundance in the world of Chainsaw Man and take their turn reincarnating and making a mockery out of death. So, one could say that Chainsaw Man brings these abominations some karmic justice.