According to friends and family, a much darker Ocampo returned home after he was discharged in 2010. His parents separated, and his father eventually became homeless.

He was arrested Jan. 13 after a locally known homeless man, John Berry, 64, was stabbed to death outside an Anaheim fast-food restaurant. Bystanders gave chase, and police made the arrest. Ocampo is being held in isolation at the central jail in Santa Ana for his own safety because of the notoriety of the case, according to Lt. Hal Brotheim, a spokesman with the Orange County Sheriff's Department.

Ocampo's father, 49-year-old Refugio Ocampo, said his son came back a changed man after serving in Iraq, expressing disillusionment and becoming ever darker as his family life frayed and he struggled to find his way as a civilian.

The son followed a friend into the Marine Corps right out of high school in 2006 instead of going to college as his father had hoped. Itzcoatl Ocampo was discharged in 2010 and returned home to find his family in disarray, his father said.

The same month, one of Itzcoatl Ocampo's friends, 22-year-old Cpl. Claudio Patino IV of Yorba Linda, was killed in combat operations in Helmand province, Afghanistan.

"Once he received the news, he was never the same," Mixcoatl Ocampo said. He said his brother visited Patino's grave twice a week.

Refugio and Mixcoatl both described a physical condition Itzcoatl suffered in which his hands shook and he suffered headaches. Medical treatments helped until he started drinking heavily, both said.

"He started drinking like crazy—too much, way too much," the father said.

A neighbor who is a Vietnam veteran and the father both tried to push Itzcoatl to get treatment at a Veterans hospital, but he refused. Refugio Ocampo said he wanted his son to get psychological treatment as well.

"He started talking about stuff that didn't make any sense, that the end of the world was going to happen," he said.