The threat of a devil and demons is a fantastic way to get people to conform to a religious doctrine. The JW org play this card all the time. If you don't do what we say you are pleasing Satan... This is so childish, so black and white! Real life is not like this.
JWs and the early christ sects for that matter, made just the same warnings. It's a psychological trick, as they say: talk of the devil and he bound to appear.
Once a person researches cult religion and its mind control, it becomes apparent that these infantile methods work on the people who do concern themselves with the devil, those who have been held back in their education and natural development. Mature humans don't worry about invisible spirits unprovable fears and threats from another dimension, the real world has enough to worry about!...but our unfounded fears can be conquered.
Life is better than you think once you have shaken off the Watchtower mindset.