Taylor, who was once close to his son, has not talked to his son in months. Young Mark and his mother live with a relative in Espanola, N.M., surviving on food stamps and disability income. When ABCNews.com talked to Mark, his speech was slurred and incoherent. His mother, Donna Taylor, was equally distraught. "We are living like gypsies," she said.
"I need help so much," she told ABCNews.com, intermittently coughing and crying. "I'm so scared. He's almost choked me several times. He cannot think logically anymore."
Mark could only manage to say, "I don't want to talk about it anymore. It's America's guilt."
His father described Mark as "paranoid and delusional," and said mental health officials diagnosed his son with a genetic bipolar disorder that was exacerbated by the trauma of Columbine.
Mark, whom police referred to as "dead kid walking," was hit when the killers aimed at students sitting on the grassy knoll adjacent to some steps. He fell to the ground and played dead while three others escaped uninjured.
He spent 36 days in the hospital and was tutored during his convalescence but never graduated from high school. "It was pretty horrific," said his father. "Basically, you're in shock."
After the shootings, Mark participated in the making of Michael Moore's 2002 anti-gun documentary, "Bowling for Columbine," and wrote a book about his inspirational recovery, "I Asked, God Answered: A Columbine Miracle."
The book logged some sales early on, but Mark was "not in a state" to promote it, said a spokesman for Tate Publishing, which released the book in 2006.
Donna and Mark Taylor, who have six other children, divorced in the aftermath of Columbine. Their son experienced several mental breakdowns and was hospitalized three times, according to both parents.
Mark's mother believes the antidepressants doctors prescribed were "killing" her son, and she took him off all medications. In 2006, they began a three-year odyssey moving from Colorado to Kansas City to live with friends. For a while, they were living in a homeless shelter in Buena Park, Calif.
Today, the 26-year-old is too sick to work and has only a Lexus to show for the estimated $185,000 he received from the Columbine victim's fund.
"He blew it all," said his mother. "It's hard to control someone that age."
Like the rest of the nation, 10 years after Columbine's massacre, Donna Taylor "cannot let this thing go," said her ex-husband.
"The mere fact is that guns aren't the only problem," said Donna Taylor. "Life is so full of darkness. School should be the safest place, not the most dangerous place for your child."
Retired teacher Kent Friesen has channeled his own grief into the plight of returning Iraq War veterans, many of whom suffer from post-traumatic stress.
"I am passionate about this," he said.
A 2007 American Psychiatric Association survey of soldiers showed overall prevalence of PTSD was 16.6 percent. But 31.8 percent of those wounded or injured reported having the disorder.
Today, Friesen cannot talk about the veterans without breaking down. "I admire those men," he said. "I hope they get the help that they need."
These days, he finds solace making furniture in his workshop and volunteering for a church youth group. Friesen cannot view violent movies without crying, but he relies on the patience of his wife and two sons, 30 and 32, to steer him clear of emotional triggers.
"I will put this [interview] on a back burner and deal with it later," Friesen told ABCNews.com. "I probably won't sleep well tonight. The thoughts and things race in my mind.
"But that's OK," he said. "That's what I have to do. I don't want to bury it, and my release is my tears."
Next: The Columbine Shootings, 10 Years After: Portrait of the Killers
ABC's information specialist Nicholas Tucker contributed to this report.