Daniel St. Hubert, who is accused of killing a 6-year-old in East New York, is being eyed for stabbing a man on a subway platform in Chelsea. View Full Caption DNAinfo/Trevor Kapp; NYPD (inset)

CHELSEA — The ex-con who police believe fatally stabbed a 6-year-old boy and knifed a 7-year-old girl in a Brooklyn elevator also stabbed a man in a Chelsea subway station Wednesday and was on his way to kill his own mother when he was caught, sources said.

Daniel St. Hubert, 27, was allegedly caught on video stabbing a homeless man at least twice in the torso on the southbound 18th Street 1 train platform at approximately 5:30 a.m. Wednesday before fleeing the station, sources said.

St. Hubert had been sleeping on a bench beside the homeless man, who witnesses said is named Kyle, when he awoke and began stabbing him, sources said.

Kyle was taken to Bellevue hospital in stable condition, police said.

Investigators initially believed that Kyle was attacked during a dispute with three men, but sources said the other men had actually been trying to help him after the stabbing.

St. Hubert has nine prior arrests, including attacks on police, a corrections officer and his mother.

He was released May 23 after serving five years behind bars for choking his mom unconscious with an electrical cord, for which he pleaded guilty to attempted murder, records show.

"Mom, I'm going to kill you," St. Hubert shouted at his mother during the 2009 attack, according to court documents.

Authorties transferred St. Hubert to the custody of the state office of Mental Health three times between his arrest and his conviction in 2012, according to city jail records.

On Wednesday night, St. Hubert was arrested near 145th Street and 133rd Avenue, just two blocks from his mother's Queens home, where sources believe he was on his way to kill her.

"We believe we prevented another murder — his mother — when we caught him," a top police official said.

St. Hubert's mother would not come to the door of her South Jamaica home Thursday, but a basement tenant said the two had a strained relationship.

"She was scared of her son," Jean Boursiquot, 63, who rents the basement of the mother's single family home.

She asked the him to talk to her son about going to a 1 p.m. meeting with his parole officer, he said.

"I see the guy laying down in the sofa no shirt sleeping. The guy was suppose to go to he parole officer at one but he was still there. I said put your f---ing shirt on and go to your parole officer or you're going to get arrested," Boursiquot said. "I was trying to help him out."

St. Hubert had been at large since allegedly stabbing Prince Joshua "P.J." Avitto, 6, and his friend Mikayla Capers, 7, inside an elevator in East New York's Boulevard Houses on Sunday afternoon.

Despite the massive manhunt for St. Hubert, who shaved his head and beard after the attacks, police failed to recognize when they issued him a summons for public urination hours before he was arrested, sources said.

The ex-con is also being eyed in the May 30 fatal stabbing of 18-year-old Tanaya Copeland near the Boulevard Houses. Copeland, a nursing student at Long Island University who played bass drum in her neighborhood marching band, was stabbed 32 times, sources said.

Copeland's mother, Rochelle, said it's been difficult for the family to cope with the ongoing uncertainty about whether St. Hubert's is the one responsible for her daughter's death.

"It just hurts a lot. I'm not relieved yet because my child['s killer] is not found out. I'm hoping I can have some closure," the grieving mom said Thursday.

But she said she was astounded that St. Hubert had tried to kill his own mother once before and was allegedly on his way to her home to try again when he was caught.

"She gave birth to you. How could you?" Copeland said.

Additional reporting by Ben Fractenberg and Gustavo Solis.