Kim De Gelder was born in 1988 in Belgium. Described by acquaintances as a quiet and introverted individual, he exhibited signs of psychological distress from a young age. At 16, he sought psychiatric help and later attempted suicide by lying on railway tracks, an attempt that was thwarted when the train stopped in time. Despite these warning signs, he was not institutionalized. In October 2008, De Gelder moved into his own apartment, distancing himself from his parents, with whom he had a strained relationship. He later claimed that his desire to commit violent acts stemmed from a desperation to break free from his parents' influence.

De Gelder meticulously planned his attacks. He practiced stabbing techniques on wooden cutouts in his apartment and mapped out potential targets using online tools. He also purchased disguises and weapons in preparation for his crimes. Initially, he claimed to hear voices commanding him to kill, but later admitted this was a fabrication suggested by his lawyer to support an insanity defense.

On January 23, 2009, Kim De Gelder entered the Fabeltjesland nursery in Dendermonde, Belgium, through an unlocked side-door, claiming he had a question. Without warning, he entered a room and began stabbing infants and staff members. He then moved to another room, continuing his attack. Two babies, both under 1 year old, were killed, along with a 54-year-old daycare worker. Several other children and adults were injured. De Gelder was armed with a knife, and reportedly wore black-and-white face paint, which the media quickly linked to the character “The Joker” from the Batman franchise—though this was later disputed by his lawyer, who claimed De Gelder simply had very pale skin.

Shortly after fleeing the scene by bicycle, he was caught and arrested by police. During the investigation, it was revealed that a week earlier, De Gelder had also murdered Elza Van Raemdonck, a 73-year-old woman, in her home.

De Gelder was later diagnosed with psychiatric disorders but was still found legally sane and criminally responsible. On March 22, 2013, he was found guilty of four murders and multiple attempted murders, and sentenced to life in prison. He is currently incarcerated in Belgium.