After the lady saved Maniac from Mars Bar, what does he feel like doing?

Maniac probably wants to talk to the lady or thank her.

Mars Bar and Maniac get off on the wrong foot.  Maniac is not afraid of Mars Bar, because he does not know the neighborhood well.  Also, color means nothing to him.  He does not care who is black or white.  Mars Bar is unable to intimidate him, and that frustrates Mars Bar.

When Maniac takes a bite out of the candy bar Mars Bar offers him,...

Maniac probably wants to talk to the lady or thank her.

Mars Bar and Maniac get off on the wrong foot.  Maniac is not afraid of Mars Bar, because he does not know the neighborhood well.  Also, color means nothing to him.  He does not care who is black or white.  Mars Bar is unable to intimidate him, and that frustrates Mars Bar.

When Maniac takes a bite out of the candy bar Mars Bar offers him, it is a total surprise to Mars Bar but Maniac doesn’t know what he did wrong.

As usual, when Mars Bar got confused, he got mad. He thumped Maniac in the chest. "You think you bad or somethin'?"

Maniac, who was now twice as confused as Mars Bar, blinked. "Huh!"

"You think you come down here and be bad! That what you think!" Mars Bar was practically shouting now. (Ch. 10)

A lady sweeping her front yard finally stops things, by telling Maniac to go back where he belongs.  The trouble is, Maniac doesn’t belong anywhere.  He ran away from his foster home with his aunt and uncle after his parents died.

Maniac does not know what to do when the lady saves him.

Maniac just stood there a minute. There was something he felt like doing, and maybe he would have, but the lady turned and went back inside her house and shut the door. So he walked away. (Ch. 10)

While it does not actually say what Maniac wanted to do, we can make an inference.  Maniac is socially awkward.  He makes friends easily, but does not follow conventions.  He likely wanted to thank her, or get to know her better, but he did not do anything.  He might have also wanted to talk to her, to explain that he did in fact belong.

Maniac doesn’t want to go back to Amanda because Mars Bar damaged the book he borrowed.  He is afraid of what she will say.  He takes off running, and later Amanda beats up Mars Bar when Maniac admits he is the one who hurt her book.