When civilization is seized by disaster—wars, civil wars, political and economic collapse—one of the casualties is human psychology. People become rapidly brutalized as a survival mechanism. Some sections of the population become totally dehumanized, and start taking sadistic pleasure in looting, torturing and killing their enemies and even strangers.
Today you take civilization for granted because everything is flourishing, but on the day when everything collapses, you will find yourself trapped in a world full of monsters. The world seems overpopulated today, but when disasters strike and civilization collapses, the population could decline catastrophically within a period of two to five years.