Religion has always wrestled with the problem of evil. And while Judaism recognizes an external enemy called the Satan or
"adversary", it is more concerned with the Yetzer Hara, the "evil impulse" or the internal devil or enemy.
Jewish religious literature goes into it in depth and Chabad acts like they have a copyright on all of that but it's really worth checking out some classical Jewish sources to see how people have dealt with the problem over the centuries. It appears to me that when you reach a certain plateau, all of a sudden a new devil or problem arises on the horizon.
The Jewish literature says the "yetzer hara" has 3 functions:
1. It tempts man
2. It brings him up on false charges
3. It condemns him to death
It's called "old foolish king" because Jewish tradition claims it starts at birth unlike the "yetzer tov" or good inclination which starts at puberty. It's also called Pharoe, angel of Esau, angel of Death, etc.
The Jewish literature (and not just Chabad's book the Tanya), say it emanates from the left ventricle in the heart. For example, in Latin "left" means "sinister". And in Norse mythology, Hela the Godess of death used to wear a cloak because her left side was dead and she needed to wear the cloak to maintain her vitality.
The Tanya calls it a "demon and a straining force" but I believe only in what's scientific. It seems to me that it's related to the deoxygenated blood (and the denatured hemoglobin in the left side of the heart) and the effect of magnetism. Someone who knows enough about biology could take all of this out of the realm of myth and into the realm of science. Any takers?