A California teenager who killed his best friend's little brother said he did it because he "wanted to see what it was like," the San Jose Mercury News reported.

William Shultz, an 18-year-old with a suspected mental illness, was arrested Sunday after 9-year-old Jordan Almgren was found stabbed to death at his home in Discovery Bay, California. Sheriff's deputies are now trying to determine a motive for the killing that took place the day after a psychiatrist told Shultz's mother he was "delusional."

Shultz explained to the newspaper there were a series of events that led to him walking into Jordan's bedroom at 3 a.m. Sunday morning with a knife.

"I wanted to see what it was like to take a life before someone tried to take mine," said Shultz, who is currently being held at Martinez County Jail on $1 million bail.

The teen said that a month ago he started seeing strange coincidences and odd messages that led him to believe the world was coming to an end, Mercury News reported. He got into a huge fight with his parents and left home for Los Angeles.

But instead he went to his grandmother's house.

Shultz said he became obsessed with the number "3" after receiving three books from his sister, uncle and father in an attempt to help him with his mental state. One of the books, a Bible, included a crossed out "forehead." Shultz linked it to a scar on his own forehead from a childhood injury.

After returning home, the coincidences continued, Shultz said, including the sighting of a stop sign near his Discovery Bay home with the word "threes" on it. He feared a civil war was coming and made plans to move to South America, but not before purchasing survival gear, including a knife.

Shultz told the newspaper he expressed his thoughts to his mother.

"I saw it as the world was sorta coming to an end," he said. "She didn't want to hear it. It concerned her about my mental state. She wanted me to see a psychiatrist."

So on Saturday, Shultz was taken to the county hospital where he spoke to a psychiatrist. For unknown reasons, the doctor sent the teen home in a taxi and later told his mother her son was "delusional," Shultz said.

He got into an argument with his mother at home and ended up spending the night at his best friend's house when she kicked him out. By 3 a.m., Shultz said he felt paranoid and couldn't sleep. His thoughts turned to murder.

"I singled out his little brother and went into his room while he slept," he said, adding that he thought of his best friend first but he was too big. "At the time, there was no feeling...I was asking myself why am I standing over him? What am I about to do?"

Shultz said he placed his hand over the boy's face before killing him, the newspaper reported. Police caught up with him at a local hospital, where he fled after the stabbing, and arrest him on suspicion of murder.

When asked if he regrets what he did, the teen said he wishes he hadn't done it. 

"I think about what if it was my little brother," he told the newspaper. 

"In that sense, I do feel bad. I do have remorse for what I did...But there's a reason for everything that happens."