The so-called homeless killer in California pleaded not guilty to killing six people Tuesday.

Itzcoatl Ocampo, a former Marine, 23, faces six felony counts of murder. Ocampo has no previous criminal record and a motive is unclear, said prosecutor Susan Price.

"It's the simplest case of evil you've ever heard of. He has no record, but he's a person who has hatred in him and had been bottling it up for some time," she said.

Prosecutors say Ocampo stabbed four homeless men in a series of killings that terrorized Orange County in December and January.

Authorities allege he also killed the relatives of high school classmate Eder Herrera. Prosecutors initially charged Herrera with the murders but released him after obtaining new evidence.

Defense attorney Randall Longwith said after Tuesday's hearing that he believes his client may have developed a mental illness after his military service in Iraq.

Ocampo did not see combat during his 2008 tour of duty, but Longwith said he worked with a medical battalion.

"We're trying to find out specifically what he saw over there," Longwith said. "The front lines in that war are everywhere and you can't not see the horrors of war."

Ocampo is being held without bail.

Based on reporting by The Associated Press. 

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