Because the human brain is programmed to use mental shortcuts. It makes surviving much easier. It's a survival tactic to label other people as "evil" or "bad" because then you don't consider them human, or as much of a human, anymore. With that train of thought, you can do anything to them - Kill them, steal from them, or any other thing that promotes your survival over theirs.
Not saying this is "moral" or "right," rather it's just the way the human brain has evolved to survive.