BY MOST ACCOUNTS, Marquis Moses was a deeply troubled teen.

He used marijuana. He skipped school so many times and got so many flunking grades that he had to repeat the 11th grade.

By age 17, he'd already been arrested twice as a juvenile - once for assault and once for drug possession.

And he lived on a seedy North Philadelphia block with a single mother who, he has said, frequently beat him and called him names.

No one foresaw that he would become a killer.

But, maybe, someone should have.

Kids who kill, experts say, share a common profile of hardship: they're high-school dropouts or troubled students; they have previous arrests; they have an uninvolved or absent father and/or were born to a teenage mother; they are involved with drugs or have relatives who are; they come from low-income neighborhoods; and they have family members with criminal records.

In a city where the number of kid killers rose 52 percent, from 19 in 2007 to 29 in 2008, experts say that it's more important than ever for youth advocates to preemptively treat the troubles that can deliver teens to their deadly destinies.

Moses made his leap from troubled teen to murderer on July 15, 2007, when he decided, as he walked with five friends two blocks from his mother's then-home, to "drop a body" and randomly punch a stranger, according to court documents.

With a single jab of his fist, Moses knocked David Cheng to the ground, according to court records. Cheng, 55, fell and hit his head on the pavement. He was declared dead on the scene at Lehigh Avenue near Germantown.

Moses later told police that he was high on drugs during the incident. In a recent letter to the Daily News, he insisted that he wasn't guilty - even though he pleaded no contest in court - and "took the case for a friend who just came home from a placement."

Moses was convicted of third-degree murder and sentenced to eight to 16 years in state prison.

His case was an eerie precursor to last year's high-profile slaying of Starbucks manager Sean Patrick Conroy, who died after five teens on a similar "drop-a-body" mission beat him, triggering a fatal asthma attack in a Center City subway concourse.

Both cases illustrate another trend in juvenile homicide: Murders by juveniles typically are carried out by pairs or packs by teens reluctant to look like wimps.

"Delinquency is, by and large, group behavior," said Phil Harris, a criminal-justice professor at Temple University. "Kids take bigger risks in a group than alone."

To some, that trend seems insurmountable.

How, for example, do you combat peer pressure and mob mentality, two teen-typical problems that are as old as pimples and puberty?

And, when so many inner-city youths grapple with hardships common to juvenile murder defendants, what makes one kid kill and another survive and thrive?

Dr. Joel Fein thinks the answer is community intervention.

Fein is an emergency-room doctor at Children's Hospital of Philadelphia, where he has treated more kids injured by violence than he cares to count.

He was on the advisory board of a program several years ago involving young patients with gunshot wounds at three city hospitals: Albert Einstein Medical Center, Temple University Hospital and the Hospital of the University of Pennsylvania.

Instead of just patching them up physically, the program paired patients with psychologists to erase the revenge mind-set that so many develop, and with social workers to secure community services that would ease their other burdens.

The program ended in 2003, when the grant ended.

But, Fein now is involved in other community-based efforts aimed at improving kids' lives before violence shatters them.

One is a 10-session after-school program for 10- to 14-year-olds that promotes leadership and positive behavior and reduces aggression. Other planned efforts include studying how greening programs and vacant properties affect violence rates.

"These kids have a very high rate of recidivism," Fein said of violently injured juveniles he has treated. "Once you have a gunshot wound, the second and third gunshot wounds are not that far behind."

Gary Server, Moses' defense attorney in the homicide case, also thinks that community intervention is key. He has represented many teens in the past 20 years and had previously worked with kids as a Philadelphia public-school teacher.

Kids "all hunger for adult human contact," Server said, "and when the kids band together to replace the adult human contact, nothing but trouble ensues."

Common Pleas Court Judge Benjamin Lerner presides over hearings in which he decides whether to transfer juveniles charged with adult crimes to juvenile court.

He says that school is a good indicator of how children are doing.

"The first canary in the mine shaft is school performance," he said. "You see a pattern of unexcused absences, suspensions and, before the dropout, a transfer to a disciplinary school."

Schools, parents and community-based juvenile advocates must aggressively address academic, disciplinary and truancy problems that could explode into criminal conduct, he added.

And while gun-rights advocates might disagree, Temple University's Harris says guns play a ruinous role.

Harris doesn't blame guns for increasing violence. Rather, they change the dynamic of conflict, he said. "Guns make conflict and violence more likely to be fatal," he explained.

Police data show 80 percent of homicide victims were shot to death, and as many as 20 juveniles are arrested every month for illegal possession of firearms, according to the Philadelphia District Attorney's Office. Still, state lawmakers repeatedly have defeated measures pushed by Philadelphia authorities to tighten gun control in Pennsylvania.

Harris recently completed a study on juvenile recidivism involving 8,000 Philadelphia youth.

He determined that juveniles who kill often fall into one of four categories: Some are so scared of their environment that they arm themselves for self-defense; others carry weapons because they're involved in drugs or other illegal enterprises; some are psychopaths with no qualms about using guns to end disputes; and others become accustomed to violence because they live with domestic abuse.

"It's very much an environmental set of factors," Harris said. "When kids have models around them, whether they be adults or juveniles, who use violence and use it successfully to gain respect or safety, then those become models the kids mimic."

But arguably the best tool to keep kids from killing might be advice from the kids themselves.

Moses, jailed at the State Correctional Institution in Houtzdale, Clearfield County, bitterly regrets his involvement in Cheng's death.

"Some situations can be stopped before things get out of hand and you lose your life or take a life, and the rest of your life, you have to spend it in jail, an [sic] nobody wants to do that," Moses wrote in neat, looping script in a letter to the Daily News. "Just when you think things can't get know [sic] worse, they can."

Staff writer Julie Shaw contributed to this report.