MILWAUKEE, Wisconsin (CNN) -- All Nathan Moore says he wanted to do was smoke pot and get drunk with his friends.
Killing Rex Baum was never part of the plan that day in 2004.
"It all started off as a game," Moore said.
The 15-year-old and his friends were taunting the homeless man -- throwing sticks and leaves -- after having a couple of beers with him.
No big deal, Moore says, but he's sorry for what came next.
It was a mistake, he said, a sudden primal surge that made him and his friends Luis Oyola, 16, and 17-year-old Andrew Ihrcke begin punching and kicking Baum.
"Luis says 'I'm gonna go hit him,' We're all laughing, thought he was joking around,'" but he wasn't, Moore concedes. "We just all started hitting him."
They hurled anything they could find -- rocks, bricks, even Baum's barbecue grill -- and pounded the 49-year-old with a pipe and with the baseball bat he kept at his campsite for protection.
Ihrcke smeared his own feces on Baum's face before cutting him with a knife "to see if he was alive," Moore said.
After destroying Baum's camp, the boys left the homeless man -- head wedged in his own grill -- under a piece of plastic where they hoped the "animals would eat" him.
Then, Moore says, they took off to grab a bite at McDonald's.
Baum's murder was indicative of a disturbing trend.
A National Coalition for the Homeless report says last year, there were 122 attacks and 20 murders against the homeless, the most attacks in nearly a decade. (Coalition report on 2006 homeless attacks)
Police found Baum's body two days after the teens attacked him.
They bragged about it around town. Police picked them up and they described what happened.
Ihrcke told police that killing "the bum" reminded him of playing a violent video game, a police report shows.
All three teens pleaded no contest to first degree reckless homicide charges and went to prison.
Moore recently turned 18 at Columbia Correctional Institution in Portage, Wisconsin, where he is serving a 15 year sentence.
"When [the beating] stops, you say, 'What did we just do?'" he told CNN. "There's no rational explanation." (Watch teen explain how "game" became tragedy Video)
Teenage 'amusement'
"It's disturbing to know that young people would literally kick someone when they're already down on their luck," said Michael Stoops, the executive director of the Washington-based National Coalition for the Homeless. "We recognize that this isn't every teenager, but for some this passes as amusement."
Criminologists call these wilding sprees "sport killing," -- largely middle-class teens, with no criminal records, assaulting the homeless with bats, golf clubs, paintball guns. (Watch how nights are cold, fearful for Milwaukee homeless Video)
Some teens have even taped themselves in the act. Others have said they were inspired by "Bumfights," a video series created in 2002 and sold on the Web that features homeless people pummeling each other for the promise of a few bucks.
A segment called "Bum Hunter," hosted by a Crocodile Hunter-like actor wearing a safari outfit, shows him "tagging" homeless people by pouncing on them and binding their wrists.
The distributors of "Bumfights" have claimed they've sold hundreds of thousands of copies.
But the company has had to deal with a couple of legal issues unrelated to the Baum case.
Last year, three former homeless stars of "Bumfights" won a civil suit against filmmakers. Santa Monica attorney Mark Quigley, who represented Rufus Hannah, known as "Rufus the Stunt Bum" to series' fans, said he is unable to disclose the amount of the settlement.
Also, in July 2006, a California judge ordered "Bumfights'" producers Ryan McPherson and Zachary Bubeck to spend 180 days in jail for failing to perform community service related to guilty pleas they previously entered to charges of staging illegal street fights.
"Bumfights" directors did not answer CNN's request for an interview.
Attacks across the nation
Incidents of teen-on-homeless violence dotted the map last year. Florida racked up at least six such attacks in 2006. (Homeless attack across U.S.)
In Lauderhill, four teens were arrested after they allegedly videotaped themselves beating, dragging, and stealing from a homeless man.
The victim has not been found, but the four face one charge each of strong-armed robbery.
Earlier this month, teens in Corpus Christi, Texas, videotaped themselves attacking a homeless man.
Commander David Torres said police arrested a 15-year-old and are looking for at least one more teenager and a 22-year-old who described on tape what they were about to do before they jumped on the man. (Read full story)
On the other side of the nation, former Oregon State University student Joshua Grimes stands accused of shooting and injuring a homeless man from his perch in a fraternity house window.
He has not yet entered a plea, but, according to a police report, he cried to detectives after the October shooting, telling them, "I didn't mean to shoot him."
At least three homeless people in Kalamazoo, Michigan, reported being attacked by teens on bicycles during a 10-day span in October, according to the homeless coalition.
In Huntsville, Alabama, six teens -- one of them 13 -- beat a homeless man with golf clubs, the coalition reported. But perhaps the most shocking of these examples was 2006's first recorded case of teen-on-homeless violence.
On January 12 in Fort Lauderdale, Florida, a surveillance camera captured two teens beating a homeless man with bats.
Prosecutors say 17-year-old skateboarder Tom Daugherty, 18-year-old Brian Hooks, a popular hockey team captain, and a third unseen teen, Billy Ammons, a high school dropout, assaulted two more homeless men that night.
One of them was 45-year-old Norris Gaynor. A witness, Anthony Clarke, told police and CNN last year that he saw the three teens approach Gaynor as he slept on a park bench. Daugherty began whacking Gaynor with a bat, Clarke said. (Watch two teenagers beat a cowering homeless man with bats Video)
As Gaynor lay dying, Ammons shot him with yellow paintballs, later remarking that the beating felt like "teeing off," police said.
Gaynor was beaten so badly his own father didn't recognize him. Facing life in prison, the teens face trial for murder later this year. They have each pleaded not guilty to one count of first-degree murder and two counts of attempted murder. (Read full story)
Lingering questions
Stoops and Brian Levin, a California State University hate crimes expert, say common themes run through teen-on-homeless violence. The attackers are almost always boys, peer pressure and mob mentality sweep away caution, and parents don't suspect their children could be capable of such actions.
Laura Simpson didn't. Her son, Justin Brumfield, is serving an 11-year prison stretch in California.
In August 2005, Brumfield and William Orantes, both 19, beat 56-year-old Ernest Adams with bats. Adams emerged from a coma three weeks later with dents in his skull, permanent scars and no vision in one eye, the Los Angeles Times reported. Orantes is serving a three-year sentence.
Simpson, a sixth-grade teacher, says she is still tormented by her son's actions and wonders if her son's irritability was more than typical teenage moodiness.
She has other questions: Was her son, a natural follower, just succumbing to peer pressure? Was he that into "Bumfights"? Did he see the fear in Adams' eyes when he raised the bat to strike him?
In a sad irony, she had adopted him; his mother was a homeless drug addict, a revelation he had learned not long before the beating and which his attorney used to explain his rage.
Her son has told her he is sorry for what happened, but her questions remain unanswered.
"As a parent, of course you're going to question yourself," she said. "It was just hard to comprehend. The first thing was, 'Not Justin. There has to be a mistake,'" she said. "You think you know everything that's going on and you don't."
When the mob mentality takes over, even the perpetrators may not comprehend what's going on.
Back at the prison in Wisconsin, Nathan Moore seems baffled by his own actions. Killing Rex Baum now registers like a "blur" or "dream," he says.
Moore and his friends knew Baum from around town. Life had been painful for the homeless man from the start; alcohol eased it. As a kid growing up in Milwaukee, when his home life became too rocky, a neighboring family took him in. He drifted through school and a brief stint in the military, his friends say, a wanderer, a loner. (Audio Slide Show: Remembering Rex Baum)
Homeless for years, he defied Wisconsin winters by constantly walking around the city, bundled in a coat patched with duct tape. For a few dollars, he pumped gas, shoveled snow off driveways, and walked neighborhood dogs.
More than 100 people came to Baum's funeral. Someone sent a newspaper clip of the story to Moore in prison.
"Every day I wish I could take it back," he said. "I seen [the] repercussions among everyone. I didn't think about any of this when [the beating] was going on."
