The worst people of all are the ones who have been absorbed in evil pursuits because of their love for themselves and whose inward behavior has been deceitful throughout. This is because the deceit wholly permeates their thoughts and intentions and infects them with venom, destroying all their spiritual life. Most of them are in the hells toward the rear and are called demons. They take particular delight in making themselves unnoticeable and floating around others like ghosts, doing their harm covertly, spraying it around like the venom of snakes. They are tortured more severely than others.
In contrast, if people have not been guileful but have been taken in by the craft of the malicious and still are engaged in evils because of their self-love, they find themselves toward the rear as well, but not so deep down. People engaged in evils because of their love of the world are in the hells toward the front, though, and are called spirits. They do not have the same kinds of evil, the same kinds of hatred and vengefulness, as people involved in love for themselves, so they do not have the same kind of malice and craft. As a result, their hells are milder.
I have been allowed to find out from experience the nature of the malice of the people referred to as demons. Demons do not flow into people’s thoughts but into their affections. They observe them and sniff them out the way dogs sniff out wild animals in the woods. When they notice good affections, they change them instantly into evil ones by using delights in something else to lead them astray in quite amazing fashion. They do this so subtly and with such malign skill that the victim does not notice anything. They take the greatest care to prevent anything from obtruding into thought because this would bring it into the open. In our case, they settle near the back of the head.
These are people who, in the world, enticed the minds of others deceptively, using the delights of their affections or their impulses to mislead and persuade them.
However, the Lord keeps these spirits away from anyone for whom there is any hope of reformation because they are the kind of spirit who can not only destroy conscience but also stir up our inherited evils that would otherwise lie hidden. So to prevent them from leading us into those evils, the Lord makes sure that these hells are completely closed off. When people of this nature arrive in the other life after death, they are instantly thrown into their hell. When they are examined as to their guile and craft, they look like snakes.
from Heaven and Hell, Sections 578-579