Demons are a very human creation. You look for ways to explain evil, and instead of seeing it in yourselves, you offload the responsibility onto monsters. The monstrous exists in the mirror, not in the sulfurous depths of some fantasy world.
The quote suggests that the concept of demons is a human construct used to rationalize and externalize the existence of evil. Instead of recognizing their own flaws and dark impulses, people prefer to attribute maliciousness to mythical creatures or external forces. This avoidance allows individuals to escape accountability for their actions.
The author emphasizes that the true source of monstrous behavior resides within humanity itself. By facing this uncomfortable truth, we can acknowledge our capacity for evil instead of projecting it onto fantastical beings. This introspection can lead to a deeper understanding of our nature and moral complexities.