WASHINGTON — Of the many questions left in the wake of Friday's shooting rampage in Colorado during a Batman movie premiere, one stands out: why did the presumed perpetrator choose not to take his own life?

Suicide is often the final act in a mass killing like the one John Holmes is presumed to have carried out. It's what happened in Montreal in 1989, Dunblane, Scotland in 1996, Columbine in 1999 and Virginia Tech in 2007, among others.

Yet after 12 people were killed and 58 wounded at a midnight screening of "The Dark Knight Rises," Holmes, 24, put up no resistance when he was arrested -- clad in body armor -- in the cinema parking lot in Aurora city.

"You have to wonder, was it redemption? Or 11th-hour remorse? Or does he need a platform" to espouse some kind of manifesto, Frank Farley, a psychologist and professor at Temple University in Philadelphia, told AFP.

The fact that Holmes chose not to kill himself, or to engage in a desperate shoot-out with police, "does seem an odd component of this particular case," added Allen McConnell, a psychology professor at Miami University in Ohio.

"His being alive might provide some insights" into the complicated web of factors and circumstances that can potentially lead someone to carry out a mass killing, he told AFP.

Dressed in a maroon prison jumpsuit, and with a shock of dyed orange hair, Holmes appeared vacant Monday at his first court appearance, his eyes in turns staring blankly or drooping shut as if he was about to sleep.

Profiles in US media described Holmes, a native of San Diego, California, as a loner who rarely if ever exchanged hellos with neighbors in his apartment building in Aurora.

If there were any points of friction with his parents, family lawyer Lisa Damiani refused to discuss them. "The family has elected not to discuss James or a relationship with James at this time," she told reporters in San Diego.

Jessica Cade, 23, an undergraduate with Holmes at the University of California in Riverside, remembered Holmes as "a very nice guy... very, very smart; a little weird, kind of like you'd expect a really smart guy to be."

But a graduate student who oversaw Holmes' internship at a Salk Institute computer laboratory at the University of California in San Diego disagreed that he was especially intelligent.

"His grades were mediocre. I've heard him described as brilliant. This is extremely inaccurate," doctoral candidate John Jacobson told the Los Angeles Times.

Holmes was enrolled as a PhD candidate at the University of Colorado, living on a $26,000 a year grant from the National Institutes of Health for promising neuroscientists, university officials said.

But in June he inexplicably withdrew from his demanding four-year program after facing a panel of professors for an oral exam and reportedly failing, Denver television station KMGH-TV said.

"I think his college experience is very important for us to analyze," said Farley, a former president of the American Psychological Association who knew one of four scholars slain at the University of Iowa in 1991 by a student who then committed suicide.

"It could be an ingredient (among many contributing factors) that he began to feel rejected," he said, noting the intense pressure that any doctoral program involves.

Shopping online over several weeks, Holmes built up a small arsenal including a shotgun, an AR-15 assault rifle, two Glock handguns and more than 6,000 rounds of ammunition, plus body armor and a gas mask, police said.

He applied to join a gun club, said its owner Glenn Rotkovich, who recalled hearing a "incoherent, just bizarre" message on Holmes' answering machine when he called him back.

Holmes also reputedly turned to Adult Friend Finder, which calls itself "the world's largest sex dating site," in search of companionship, sporting the same orange hair in his profile photo as he did Monday in court.

"Will you visit me in prison?" said the profile, dated July 5, according to gossip website TMZ, which said it had spoken with three women who had turned down requests from Holmes to meet up.

"One of the women tells us ... Holmes wasn't aggressively seeking out sex -- in fact, she says he was 'just looking to maybe chat ... nothing sexual'," said the website.

ABC News meanwhile obtained a video made at a San Diego science camp six years ago in which Holmes, then 18, looking slightly nervous, delivers an oral presentation to his peers.

"Over the course of the summer I've been working with a temporal illusion," he said. "It's an illusion that allows you to change the past."