Kai, whose real name is Caleb Lawrence McGillvary, was captured at the Greyhound Bus station in Philadelphia, WABC-TV, New York, reported.
Police said McGillvary is responsible for the death of lawyer Joseph Galfy, who died of blunt force trauma at his Clark home, Union County Prosecutor Theodore J. Romankow said.
McGillvary and Galfy were seen in video footage near New York's Times Square, and the men later went to Galfy's house, police said.
Before his arrest, McGillvary had said on Facebook he was raped by Galfy.
McGillvary, a 24-year-old homeless man who relies on strangers for food, housing and transportation, gained fame on Facebook and YouTube for intervening in a Feb. 1 attack in Fresno, Calif. The driver of a vehicle in which McGillvary was riding had hit a utility worker and then got out of the vehicle to confront the worker. McGillvary then hit the driver with a hatchet.