Malik Vincent Murphy, the Colorado man accused of stabbing his younger brother and sister to death as they slept, allegedly told investigators he thought about killing his family for months, according to multiple reports.

In a three-hour long court hearing Friday, prosecutors played a video of Murphy, 20, allegedly confessing to police, ‘I can’t stop thinking about killing people,” KMGH-TV reports.

Murphy is accused of attacking his younger siblings and father with a hunting knife on Oct. 17. He has been charged with two counts of first-degree murder in the deaths of his brother Noah Murphy, 7, and sister 5-year-old Sophia Murphy.

He is also charged with attempted first degree murder for allegedly stabbing his father, who went to check on Sophia after hearing her screams, in the neck.

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Murphy allegedly told investigators he had thought about killing his family for months. Saying it was “something that had to do,” KMGH-TV reports. When asked why, Murphy allegedly replied, “to be by myself.”

During the hearing, prosecutors discussed Murphy’s alleged history of violent thoughts and mental illness, which included being treated at local psychiatric facilities multiple times, according to the Colorado Springs Gazette.

Two years before the alleged October attack, Murphy allegedly hid in the bathroom of his Lincoln, Nebraska, high school with a plan to strangle the next person who came in, the Gazette reports. He never carried out his plan due to his nerves, he told investigators.

The hearing was attended by Murphy’s parents, who sat in the front row and were visible upset, according to the paper.

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Murphy’s public defenders plan on using a mental health defense, according to the Gazette. Video footage of Murphy talking to himself during questioning shows the the suspect repeating to himself, “no, no, no,” “Oh my God, why?” and “Malik, what did you do?”

Murphy told police he had been diagnosed with anti-social personality disorder and depression, the Gazette reports. He allegedly admitted to not taking his medication regularly and having violent thoughts against his family.

Murphy’s next scheduled court appearance is in April. His public defender could not be reached for comment Monday.