[3] Yoo had hoped to attend an arts high school, but after experiencing the humiliation of a color-blind[4] interview, he spent his time fiddling with drawing pencils instead of paints, developing an eccentric personality.
[5][6] From September to November 2003, Yoo killed several wealthy senior citizens by breaking into their houses and bludgeoning each of them with a hammer.
[7] On 19 July 2004, Yoo revised his confession, admitting to a further six murders, raising his tally to 26 individuals, although no clarification or explanation was offered at that time.
The list of purported victims included several individuals who did not match his prior pattern of wealthy seniors or masseuses.
[9] Several days later, Yoo also confessed to killing a young woman (a worker in a clothing store) on 6 February 2004 in Imun-dong after he suspected her of being a prostitute.
[1][15] Yoo was taken into custody on 15 July 2004 and confessed to murdering as many as 19 people initially, specifically targeting affluent senior citizens and masseuses.
[17] The mother of the Imun-dong murder victim rushed at Yoo with an umbrella when he was brought to the prosecutor's office later in July, screaming that her daughter would still be alive if the police had captured him earlier.
[18][19] Based on the contents of his apartment, searched after his arrest, there was some speculation that he patterned his killings after several movies, including Public Enemy, Very Bad Things, and Normal Life.
[15] A psychologist who assessed Yoo concluded that he was not mentally ill, but exhibited the typical characteristics of anti-social disorder whereby a person creates their own belief system based on a rejection of moral and social norms.
[21] Yoo first appeared in court on 6 September 2004, refusing to defend himself, declaring his intention to boycott the remainder of the trial and apologizing to the victims.
[35] In the early days of his sentence at Seoul Detention Center, Yoo was not required to perform hard work, when food came in he ate the biggest and tastiest part, he knocked on the doors at 10 or 11 at night to harass the guards and made them boil water for his coffee, and acted recklessly.
[37] In November 2014, it was confirmed that he had been smuggling in Japanese comics, adult picture books, and erotic novels by bribing prison guards.