Though feeble-minded, Lennie is aware that he has done something irrevocable, something terrible and something which condemns him as "bad." His Aunt Clara had been the one to look after him as a child, and now the image of her looms before him in reprobation. This image transforms into a giant rabbit, still condemning him for his transgression, for crushing the life out of small things entrusted into his care.
Lennie instinctively knows now that he is not trustworthy enough to look after rabbits, or anything else for that matter. He cannot differentiate between the accidental killing of Curley's wife and that of the puppy or mouse; on all accounts, he feels guilty and worthy of punishment. His reverting to a childhood experience is own kind of defense mechanism to somehow reduce the wrong he has done down to a scale he can handle. He wants to be scolded as a naughty boy for a bad prank he has done, but deep down he knows this time he's guilty for a crime he cannot "uncommit."