SANTA CRUZ — When they first got word that he was a murder suspect, those who watched 15-year-old Adrian Jerry “A.J.”Gonzalez grow up before their eyes at the Tannery Arts Center held out hope. It must have been a mistake, or at worst an accident, they thought.

But the image of the mild-mannered yo-yo slinger zipping around on his skateboard quickly started giving way, first with the public surfacing of social-media posts portraying a potentially troubled and suicidal teen, then with the revelation that he had obsessively asked neighbors about the search for the girl he is now accused of killing.

For many, whatever benefit of the doubt remained crumbled Wednesday when the Santa Cruz County District Attorney’s Office announced Gonzalez would be prosecuted as an adult for murder, kidnapping and sexual assault, based on allegations he lured his 8-year-old neighbor Madyson “Maddy” Middleton to his family’s third-floor apartment Sunday, tied her up, beat, raped and strangled her, then dumped her in a recycling bin in the parking garage downstairs.

Gonzalez, a Santa Cruz resident and one-time counselor at the artists’ residential community, is set to be arraigned Thursday, and faces life in prison if convicted. Minors are not eligible for the death penalty.

Among the Instagram portraits of him outside the Tannery and showing off yo-yo tricks in an apartment were other, darker pictures and commentary. Most chilling was a post from Monday, the second day of the search and the day Maddy’s body was found, that showed a short black-and-white clip of hands playing a piano to the tune of Gary Jules’ version of “Mad World” — popularized on the soundtrack of 2001’s “Donnie Darko,” a film signified by the title character’s death dreams.

The caption quotes the song’s signature line: “The Dreams in which I’m Dying are the Best I’ve ever had.”

Psychologists and a nationally-renowned child sex crimes expert who reviewed the teen’s posts for this newspaper differed on how much concern his online behavior warranted before the killing.

“It definitely sounds like this guy was dealing with depression and thinking about suicide,” said San Bruno-based clinical psychologist Oscar Cervantes. “It seems like this guy knew something was really wrong with him.”

Other posts illustrate a teenager vacillating between being lively, sullen and appearing to reach out to someone — one image of him standing in an open field is captioned, “In solitude, you’re the only one I want beside me.”

“Through the lens of knowing about these horrible acts, you look at it completely different than if it was someone else’s 15-year-old son,” said Cordelia Anderson, founder of the National Coalition for the Prevention of Child Sexual Abuse and Exploitation. “If we weren’t looking through that lens, would it say anything? That age is full of angst.”

Dudley Luke, a Palo Alto-based psychologist who specializes in adolescents, said a post where Gonzalez acknowledges a “crippling anxiety” offers its own clues.

“What’s counter to a sociopath is that a sociopath wouldn’t write about his weaknesses,” Luke said. “He’s got some insight, but a sociopath doesn’t show himself as a victim.”

Late Wednesday morning, Gonzalez’s Instagram account was taken offline, after it was flooded with web traffic and threatening comments.

Maddy was found dead about 7:55 p.m. Monday in a recycling bin in a ground-floor parking garage at the Tannery Arts Center, following a massive search triggered by a 911 call just after 6 p.m. the previous evening.

It was during that search that Tannery resident Setorro Garcia said he practically had to shoo away Gonzalez because of his incessant questions about the status of the search. At one point, Garcia said he told the teen: “Dude, you’re the only one asking me.”

“We’re confident in the charges we filed and that’s what justice demands,” said Santa Cruz County District Attorney Jeff Rosell.

He added: “I can’t remember any 15-year-old with these specific charges” that include six felonies, among them murder, forcible rape, great bodily injury and kidnapping by lying in wait.

While news reports have suggested a sadistic impulse spurred the killing, authorities say they are awaiting ensuing court hearings to formally release a motive or any other details that might shed light on a death that has rocked Santa Cruz to its core. Police have interviewed Gonzalez extensively.

“People do things for lots of reasons. Sometimes we understand them. Sometimes we don’t,” Rosell said.

Such understanding has been hard to come by in the past 24 hours among the tight-knit Tannery community.

“He was the quiet kid that played with the yo-yo,” Garcia said.

Lisa Silas, who has lived at the artists’ residential complex since it opened in 2009, is struggling to piece together the pleasant boy she knew and the killer that police say he is.

“First I’m very, very heartbroken for Maddy. But I thought it had to be a mistake,” Silas said, while fighting through tears. “This is really hard. We’re just really upset, really devastated.”

“We don’t know what happened. This may have been there, but we didn’t see it,” Silas added. “I can’t speculate for A.J. or his mental health history, but to do something like that, he must have had some history we didn’t know about.”

Prosecutors declined to say whether Gonzalez — whose last name is spelled “Gonzallez” on his Instagram account and the Tannery website — had a juvenile criminal history, which would not be public record.

Residents, as they sort through their shock and sadness, are trying to channel that grief into support for each other. Counseling has been made readily available at the Tannery, and Silas hopes a protracted effort to install a playground for the complex’s children can find new momentum as a tribute to Maddy.

Said Garcia: “We are a family. We’ll stick together.”

Contact Robert Salonga at 408-920-5002. Follow him at Twitter.com/robertsalonga.