Levi Aron, 35, was charged with murder in the death of Leiby Kletzky, whose feet were found in a plastic bag in a freezer in Aron's home, the New York Daily News reported Friday.
The rest of the child's remains were found in a dumpster 2 miles away.
"It hurts too much to think about it," said Aron, who some have called the "Butcher of Borough Park."
"I don't know what happened," Aron said in an interview from the infirmary at Rikers Island, where he is being kept on a round-the-clock suicide watch. "I just panicked."
Investigators said the boy stopped Aron for help after becoming lost upon leaving a day camp July 11 and that the man took him to a wedding in upstate New York and then to his Brooklyn home.
Asked why he took the boy, Aron reportedly said: "He looked familiar. I thought I knew him."
Police said the boy didn't appear to have been sexually molested. Aron was declared fit to stand trial although some psychologists said he is on the edge of schizophrenia.