There are three types of people in this world: sheep, wolves, and sheepdogs. 

Some people prefer to believe that evil doesn't exist in the world, and if it ever darkened their doorstep, they wouldn't know how to protect themselves. Those are the sheep. 

Then you've got predators who use violence to prey on the weak. They're the wolves. 

And then there are those blessed with the gift of aggression, an overpowering need to protect the flock. These men are the rare breed who live to confront the wolf. They are the sheepdog. 

(in American Sniper movie, Wayne Kyle to his sons)