Propaganda is a powerful tool. The watchtower know this to be effective, therefore they use the fear of an evil entity that may or may not exist to inculcate the rank and file into believing anyone who leaves the "truth" are demon possessed.
Get someone to believe good and evil exists in the form of invisible spirit entities then you have them.
Unfortunately this does not address the issue that their are certain amounts of good and evil in all of us, it's how we control the dark side we have that determines whether we go on to be murderers or a decent member of society. I don't go for "it must have been a demonic force that made that person murder all those people" it takes responsibility away from the person who committed those evil acts.
The watchtower uses such propaganda to take the responsibility away from them, when in fact it is the watch Tower who made this "law" to protect from anyone who knows the real truth about the organization. The rank and file are essentially victims to the propaganda and most of the time cannot be blamed for something that is constantly drummed into the rank and file.