Jeannette Hemenway sets a candle at a memorial for Jordon Almgren in front of Timber Point Elementary School in Discovery Bay, Calif., on Monday. (Katrina Cameron/Bay Area News Group)

Eighteen-year-old William Shultz was paranoid about the world “coming to an end.” He was also curious about what it would be like to kill somebody. He told a reporter: “I wanted to see what it was like to take a life before someone tried to take mine.”

During a sleepover on Saturday night, in a quiet subdivision near the Sacramento-San Joaquin River Delta in Northern California, Shultz said, his mind started to race after everyone else was asleep. In the early morning hours, he said, he thought about killing his best friend but decided it would be too difficult.

The friend was too big, Shultz said, and too strong.

His plan shifted.

“I singled out his little brother and went into his room while he was asleep,” Shultz said Monday in an exclusive jailhouse interview with the Contra Costa Times. “At the time, there was no feeling. … I was asking myself why am I standing over him? What am I about to do?

“I put my hand over his face, and I stabbed him.”

Shultz has been charged with murder; police in Contra Costa County say they have not yet determined a motive.

Prosecutors say he killed 9-year-old Jordon Almgren, or “Jordy,” a third-grader now remembered fondly as the smallest kid on his baseball team with the biggest personality, according to the San Francisco Chronicle. The horrific tragedy shook Discovery Bay, a community some 60 miles from San Francisco.

“This is just hard. It’s unimaginable,” Raquel Otis, a friend of the Almgrens, told the San Jose Mercury News. “The pain and the horror of the situation is paralyzing.”

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William Shultz (Contra Costa Sheriff’s Office)

Concerns in Shultz’s home started to swell last week, according to reports. He became fixated with the number 3 and started seeing signs and “coincidences” everywhere. He noticed “strange human behavior.” He bought a Bear Grylls survival knife. He worried about civil war in America and planned to make his way south, he told the Contra Costa Times.

“I saw it as the world was sorta coming to an end,” he told the newspaper. He said he told his mother the same thing. “She didn’t want to hear it. It concerned her about my mental state. She wanted me to see a psychiatrist.”

On Saturday, Contra Costa sheriff’s deputies were called to Shultz’s home. His family was worried about his mental health, police spokesman Jimmy Lee told The Washington Post. Deputies determined that he didn’t meet the criteria needed to commit him into the county hospital because he wasn’t deemed a danger to himself or anyone else. But he agreed to go to hospital for evaluation anyway, Lee said.

At some point later that day, Shultz was sent home. After he got there, according to what he told the Contra Costa Times, he got into an argument with his mother and went to his best friend’s house to spend the night. Several hours into the sleepover, he said, he became paranoid. Then, he said, he killed his friend’s little brother.

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Early Sunday morning, the Contra Costa Sheriff’s Office got a call from the Almgrens’ home; by the time deputies arrived, a family member had already taken Jordon to a nearby hospital, where the 9-year-old was pronounced dead.

Shultz wasn’t at the house either. During the alleged stabbing, Shultz told the Contra Costa Times, he accidentally cut his own wrist. Authorities say he stole some car keys and took a vehicle from the Almgrens’ home to a hospital in Antioch, about 20 miles away. Shultz has also been charged with residential burglary.

On social media, the sheriff’s office identified Shultz as a suspect in the boy’s death and asked for help tracking him down. Lee said a nurse at the hospital in Antioch recognized Shultz from reports and notified police. He was arrested and taken to Contra Costa County Jail.

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Shultz has been charged with murder and special circumstances, though prosecutors have said they will not seek the death penalty.

In his interview with the Contra Costa Times, Shultz locked eyes with the reporter and cracked a smile.

When asked about the fatal stabbing, he said he felt bad.

“I wish that I hadn’t. I think about what if that was my little brother,” he said. “In that sense, I do feel bad. I do have remorse for what I did.

“But there’s a reason for everything that happens.”