Robert McCall is a nice guy. He challenges people to succeed with "Progress, not perfection," and he doesn't shame them for their failures. He befriends people whom others reject or ignore, like an overweight employee and a young prostitute named Teri. He's a great boss, a good neighbor, and a voracious reader of classic literature. He's also - unbeknownst to them - a mix of
Repairman Jack, Jack Reacher, and Liam "I Have A Very Particular Set Of Skills" Neeson.
One night, during one of the frequent times his path crosses with Teri's at a diner, Robert asks her why she hasn't pursued a music career.
"You and I know what I really am."
"I think you can be anything you want to be."
"Maybe in your world, Robert. It doesn't really happen that way in mine."
"Change your world."
If only life were that simple. When it becomes painfully clear that her world will kill her, he decides to change it for her. The bad guys drag other people for whom he cares into the conflict, and he unleashes a one-man war of vigilante justice. In spite of his anger, he gives those deserving of judgment a chance to do the right thing.
He might do justice, but he offers mercy. Most of them refuse it. It's a bad call on their part, because Robert is going to make sure they reap what they sowed. He stays true to his life philosophy: "When somebody does something unspeakable, you do something about it, 'cause you can."
He's a vigilante knight in shining but tarnished armor, fighting for good in a world that believes his kind of noble warrior only appears in fictional books. Teri sure doesn't think men like him exist when she meets him. She believes by the end of the movie.