Itzcoatl Ocampo was found ill in his cell Wednesday night and taken to a hospital in Santa Ana, Lt. Jeff Hallock with the Los Angeles County Sheriff's Department told KNBC-TV, Los Angeles. He died Thursday.
Ocampo had set a goal of slaying 16 people, a transcript of his interview with police indicated.
Ocampo, 23, of Yorba Linda, Calif., told detectives he knew he "had the killer gene," which is why he joined the Marines. The transcript, which was presented to grand jury members, highlights Ocampo's disappointment in not engaging in combat during his six-month tour of Iraq in 2006.
"It is the stuff that movies are made of," prosecutor Susan Price told grand jurors. "Because rarely do you find anyone that is so evil, so sophisticated, and so determined to end another person's life."
Ocampo was indicted in February 2012 by a grand jury in the stabbing deaths of four homeless men in December and January. He was arrested Jan. 13 as he ran from an Anaheim, Calif., parking lot where the fourth homeless victim was stabbed to death.
Following his arrest, he became a suspect in the deaths of a mother and her son who were killed Oct. 25 in Yorba Linda. He faces six counts of murder.