The verb pictures a man who abandons a person when he is in trouble and in need of your help. Paul was in trouble, in jail, facing execution. So Demas abandoned him. Demas jumped ship at the first opportunity. He loved the world.
Unfaithful men are selfish men, individualists, whose world begins and ends with themselves.
They have no loyalty but to themselves. They have no commitment but to themselves. They are committed to and loyal to their own personal advancement, benefit and safety regardless of the cost to others. The unfaithful stand in sharp contrast to our Lord Jesus Christ whose commitment was and is to us. This is why He was a faithful high priest.