Slay Demons to Grow Stronger: From Constable to Saint

This book is also known as "REN Demons Must Die"

The False Demon-Slaying Guard: Works diligently, slays demons, and prays for a world without demons.

The Real Demon-Slaying Guard: A world without demons? Then I'll raise them myself and let monsters and demons run wild!

Gu Ming transmigrated into a world overrun by demons and unexpectedly awakened the Demon-Slaying Manual. By devouring the lifespans of demons, he could burn his own years of life to advance his cultivation techniques.

With one Demon-Slaying Manual, he would wipe every demon and monster from the world.

All things under heaven that possess sentience can become demons.

Humans are the foremost of all living things, but those whose hearts breed monsters can also be called demons.

If you're a demon, then you must die!

But what if I am the greatest demon in the world?

Then who can kill me?

(PS: The protagonist's personality is, well, a little twisted. If that bothers you, don't read.)