when did aunt polly realise what was cruelty to the cat was cruelty to the boy too?

Tom explains for his actions by saying that he "done it out of pity for [the cat] ­ because he hadn't any aunt" to "burn him out" and "roast his bowels." Polly suddenly feels remorse, seeing that her endless doses of medicine were as much torture for Tom as it was for the cat and the two come to an unspoken understanding.