The family's once thriving T-shirt and tank top manufacturing business in the downtown Los Angeles garment district was failing, so much so that he went around to fellow merchants asking for loans to pay the rent.
Yun, 54, closed the business about two weeks ago. He and his family once lived in a house in Hancock Park but moved to Monterey Park after experiencing financial problems.
He was arrested in 2004 on suspicion of hitting his wife, Sun Ok Ma, who filed for divorce last week.
He complained to friends about the high cost of having one of his children in a Wilshire Boulevard private school. He remarked about how stressful it was for him to keep up with the wealthy parents at the school.
Authorities said that Yun told his wife on Sunday that he was taking Ashley, 11, and Alexander, 10, to a movie and would return a few hours later. Around 4:40 p.m., he drove his Toyota Sequoia into a deserted alley near his former business, they said.
Los Angeles Police Department detectives believe he splashed fuel around the interior of the vehicle.
Witnesses told police they saw him arguing with Ashley outside the car before forcing her into the back seat. They said they watched in horror as Yun climbed into the front passenger seat and the SUV erupted into flames.
Seconds later, he rolled out onto the ground, his legs engulfed in flames. Firefighters arriving at the scene found that the vehicle had burned to its frame and the children had died. Yun was under guard and listed in critical condition at Los Angeles County-USC Medical Center with burns to his hands, face and legs. He is expected to survive, police said.
"We see horrific crimes, but this is a particularly sad one and very hard to understand," said Lt. Don Hartwell.
People in the garment district, Koreatown and beyond were struggling Monday to understand how this could have happened. Shop owners who knew the family wept as they thought back to possible signs of what was to come.
"He must have momentarily snapped," said K.C. Min, who with her husband, Y.K. Min, operates a clothing business two doors down from a retail store run by Ma.
"I can't believe it. How can something like this happen? They're such nice people and they have beautiful children."
Several years ago, the couple seemed to friends to be a model for success. Their business was booming, he drove a Mercedes and the family had attained the dream of many Korean Americans: a home in the Hancock Park area near good schools for their children.
But a few years ago, when state and federal tax officials ordered them to pay $100,000 in back taxes, their marriage worsened, the Mins said.
The couple borrowed money from them and always paid it back, they said.
"Whenever Mr. Yun returned money, he would always bring us food to thank us," said K.C. Min. "He had such good manners."
Yun had earned a law degree in Argentina before immigrating to the United States in the late 1980s, the Mins said. He soon met Ma, who was a real estate agent, and they married in 1993.
Two years ago, he pleaded guilty to beating Ma and was sentenced to two years' probation. He has since been receiving counseling at a Koreatown social services group.