The door to the open prison is locked. I have to ring the doorbell and hand over my ID card to be let into the outbuilding of the prison. There, prisoners are allowed to move around freely; during the day, they can go to work or to school. A slim young man comes toward me on the staircase; I am told to call him Kai. He smiles and shakes my hand. "I will take you to Ms. Mohr's office," he says. When he was 16 years old, this friendly person wanted to kill many people.

The office of the prison psychologist, Heike Mohr, smells of fruit; a bowl of biscuits has been provided. Kai sits down on a wicker chair. He has been serving a four-and-half-year prison sentence for attempted murder and thinking a lot about what had led him to plan a violent crime.

Explosives in the bedroom

Flashback to an evening in January: Kai called the police to report a disturbance of the peace. He lay in wait for the police in the dark and attacked them with a crossbow and machete so that they would be forced to shoot him. "Suicide by cop" was something he had read up on. But the bolt lodged in a protective vest and the machete just grazed its victim. The police officers were just slightly injured and reacted calmly. They used their flashlights to blind the attacker and yelled, "Get on the ground!" Kai surrendered.

It quickly became clear that the boy had other plans: explosive devices were found in his bedroom. The police had the entire apartment building evacuated until all devices were defused. Kai had found instructions on the Internet on how to build nail bombs and Molotov cocktails. He recalls, "In the evenings, when my mother was out, I sat in front of the TV and extracted gunpowder from New Year's Eve fireworks."

Investigators recovered deleted files on his computer and found notes on a planned killing rampage. It was clear that the high school student wanted to kill many people: at school, in the town's pedestrian zone and in a café. "I changed the plans a hundred times," says Kai. At his trial, his notes were read out loud for hours. It was hard to endure, says the now 19-year-old. He had to "close his eyes and ears."

Target of ridicule

Most prisoners in the penitentiary have short hair, bulky muscles and like rap music, unlike Kai. Now he can tolerate being different. "Everyday life in jail is already therapy, like the conversations with other inmates. They ask me why I stutter; they listen when I explain. Then it's over and dealt with."

Stuttering and ridicule were part of Kai's life from a young age. During a holiday on the North Sea coast, another boy once dumped a bucket of mud on his face. In primary school, the other children mocked him. He could not defend himself verbally; violence was not an option, he says. Things got worse in secondary school, where even teachers made fun of his stutter.

Kai felt at ease in his first secondary school but his academic performance plummeted at the next school, which had a business focus. The youth was stressed out by the long commute to school, his daily stutter therapy, the great amounts of homework and the pressure to achieve. He felt like "a nothing" in this system.

First-person shooter, sleep deprivation, paranoia

He also lost important mainstays in his life, like the archery classes taught by a dedicated teacher. Kai felt misunderstood. He became increasingly withdrawn. His single mother came home late from work and his father was absent as long as he could remember. Kai spent all his time at his computer, surfing on the Internet and playing first-person-shooter games. The worse he felt, the more aggressive the games became in which he played the strong hero. Kai says that at the time, his personality was split into the "normal" Kai, the Kai who wanted to die and the Kai willing to use violence. The violent one was gaining more and more control.

He recalls how he barely slept. When he switched off the computer at night, he only had two hours before the alarm went off. He lay in bed and listened to music. Sleep researchers warn that sleep deprivation can lead to paranoia: Kai felt like he was being followed and never left home without a knife.

Even teachers' attempts to provide him with extra help were interpreted as offensive. He felt like he had done everything within his power and wondered why nobody noticed it. He could not be dissuaded from believing that everyone was against him. "I was waging a war on myself," says Kai. He wanted to die, so he slit his wrists, but not deeply enough because he was not able to. He bandaged the wounds and did not tell anyone about it.

Before anyone else had the opportunity to attack him, his paranoia compelled him to plan a "pre-emptive strike." He dreamt of a "grand exit with a big bang" and wanted it to be as spectacular as possible, like the Columbine massacre in the USA. Later on, in prison, the psychologist, Heike Mohr, gave him a book to read by the German journalist Joachim Gaertner. The book is a documentation that contains journal entries written by the Columbine assassins and is called, "I am full of hate and I love it."

"That is exactly how I felt," says Kai. He says he enjoyed "the power that goes along with hate and violence." That power represented "the antithesis of the helplessness you actually feel." The student wanted to "eradicate bad people in the world."

To-do list: Declaration of love

Many people "I wanted to get" were supposed to go to a party at a café, he says. As soon as his decision was finalized, he calmed down. Kai began building bombs. He ordered a crossbow, machete and knife from an online shop. He did not have any firearms; he wanted to get hold of them from the police.

A girl's name was written on his to-do list. He had been in love with her for years and she was "the only person apart from my mother with whom I had never stuttered." But Kai's feelings were not reciprocated and he decided to stick to his rampage plans. After the Christmas holidays, he skipped school and focused on his preparations.

Kai's mother was worried about him and feared that he would hurt himself, so she thought of going to a psychologist for counseling. But friends and relatives talked her out of it, as they claimed that behavior like Kai's was normal during adolescence and that she should stop acting like such a "mother hen." Does Kai have anything to say to concerned parents? He is convinced that, "You should not let yourself get talked out of a gut feeling like that," saying that this applied to all people in close contact with a troubled young person.

Cracks in the plan

Kai's mother recalls the evening before the deed: Her son was affectionate, like he had once been as a small child. The next day she asked him what was wrong with him and he cried. He said even he did not know. He just wanted her to go away and promised to meet her later in town. Later – that meant he wanted to be dead by then.

His mother's persistence stirred something inside of him, says Kai today. His plan to commit murder did not immediately fall apart, but it did show cracks. When he was alone in the apartment, he reached for his sports bag to pack the explosives. Suddenly, a thought came to him: "You can't do that." The moral principles he had learned in his early childhood resurfaced. "The last fuse held out, just like in an electrical circuit."

At a loss, Kai started running around the apartment. He decided to scratch the "rampage" part of the plan. He wanted to die and the police were to shoot him, but he survived because of their calm response. He now says, "They did everything right." Kai apologized to them in court.

While in prison, Kai took part in a therapy program and completed vocational training as an electronic technician for industrial engineering plants. He is currently going to night school to obtain his high school diploma and would eventually like to go to university. He is fascinated by math and physics. In the evening, his mother usually drives him back to prison. Kai says, "I am lucky to have a mother who does so much for me."

'I regret the damage'

Psychology and social work have become new interests ever since Kai has been working for a prevention program. He wants to keep others from committing violent crimes. Once, at a special needs school, he advised teachers who were concerned about a 12-year-old with extremely violent fantasies. During the conversation, Kai recognized his own motives. The boy's mother thought everyone was against her son. "She was like me back then," says Kai.

Kai continues to learn about his past: in his prevention program, in his interaction with psychologists like Heike Mohr and even in the interview with Deutsche Welle. Does he have any regrets? At first, he rejects the term, finding guilt to be a better word. Later, on the way to night school, we returned to the subject. "I only regret the damage I have caused to others," explains Kai. "I do not regret the way I have turned out. I have become what I am because of what happened. Of course, something extreme like that shouln't have to happen."

After lengthy discussions with Kai, an external examiner decided to authorize his early release. The judge agreed with her and Kai has been free since early March.