In "Thank You, M'am," what is Mrs. Jones' motivation?

Teaching Roger life lessons is Mrs. Jones's motivation.

Mrs. Jones is motivated by wanting to provide instruction to Roger.  When Roger's attempt to rob her fails because he falls flat on his back with his legs in the air, Mrs. Jones prevents any effort at escape by delivering a swift kick "right square in his blue-jeaned sitter." Not mentioning police, she drags him bodily home with her

  After that the woman said, “Pick up my pocketbook, boy, and give it here.” She still held him. But she bent down enough to permit him to stoop and pick up her purse. Then she said, “Now ain’t you ashamed of yourself?”
  Firmly gripped by his shirt front, the boy said, “Yes’m.”
  The woman said, “What did you want to do it for?”
  The boy said, “I didn’t aim to.”
  She said, “You a lie!”
  By that time two or three people passed, stopped, turned to look, and some stood watching.
  “If I turn you loose, will you run?” asked the woman.
  “Yes’m,” said the boy.

When Mrs. Jones takes Roger to her home, she provides instruction on a variety of levels.  She instructs him on the importance of personal hygiene: She makes him wash his face.  Mrs. Jones teaches Roger the lesson that others have suffered from want because of limits on money, and that not everyone wins out over temptation: 

The woman was sitting on the day-bed. After a while she said, "I were young once and I wanted things I could not get." ... The woman said, “Um-hum! You thought I was going to say but, didn’t you? You thought I was going to say, but I didn’t snatch people’s pocketbooks. Well, I wasn’t going to say that.” Pause. Silence. “I have done things, too, which I would not tell you, son—neither tell God, if he didn’t already know." 

Mrs. Jones is motivated by compassion because his actions are not unfamiliar to her: She has at one in her life behaved the way Roger has behaved.  Finally, Mrs. Jones is motivated by wanting Roger to learn before it is too late the ultimate lesson of the need to live a better life.  

When Mrs. Jones leaves Roger in the hallway with the stern warning of "Behave yourself, boy!" it is clear that Roger has learned the lessons Mrs. Jones was motivated to provide, lessons founded in understanding and compassion and mercy. 

  The boy wanted to say something else other than “Thank you, m’am” to Mrs. Luella Bates Washington Jones, but he couldn’t do so as he turned at the barren stoop and looked back at the large woman in the door. He barely managed to say “Thank you” before she shut the door.