My daughter Renske met her boyfriend Samarie on the train. She was heading from the Netherlands to Switzerland; he was an asylum seeker from Benin. They got chatting and exchanged phone numbers. That was how it started. They had a good relationship. He was attentive and they were very respectful towards each other. They spent holidays with me and my wife Lieuwkje.

Just before midnight on 13 April 2011, I saw on the news that a girl had been killed in Baflo, where Renske lived. About an hour later, they showed a picture of the scene, and I recognised her flat. I called the police and said, “I think my daughter is the victim of the incident in Baflo.” At 5am, two officers came to the house and we learned what had happened.

Samarie had picked up a fire extinguisher in the hall and beaten Renske to death. When an officer tried to arrest him, he grabbed his pistol and shot him dead. Samarie was then shot by police five times and taken to hospital. I couldn’t believe it; in the two years that we’d known Samarie, I had never seen him lose his temper. It was so far removed from the man we knew that we couldn’t make sense of it. It was immediately clear to my wife and I that he needed help, and we wanted to try to be there for him; to understand what had happened.

Initially, I thought it was connected to Samarie’s asylum claim. The day before the killing, his final appeal had been rejected and he was told he was being deported. Five weeks later I went to his flat and found a strip of pills. I knew Samarie was taking antidepressants, but I didn’t know what sort. The label said paroxetine, which is a selective serotonin reuptake inhibitor (SSRI), and as a psychologist I knew there are risks associated with it. From talking to Samarie’s psychiatrist, I discovered that he was reducing his dose. I read that in a small number of cases there have been severe side-effects to SSRIs, including outbursts of extreme violence, usually when the dose is being changed.

After a month or two, we contacted his lawyer, but we couldn’t see Samarie because he was still in the prison hospital. We wrote him a letter and he replied saying how sorry he was. In September we visited him for the first time. We wanted to see for ourselves that he was genuinely remorseful. Samarie came into the room in tears, and he and my wife Lieuwkje hugged each other. I shook his hand. We didn’t talk a lot. He was still limping from his injuries.

From then on, we visited once a month. At first, we talked about what had happened with Renske. He said he’d been in a state of anxiety all day and had tried to get help. They had an argument and she tried to stop him walking out; that was when he hit her. When I asked if he had an explanation, he said: “No, you know how much I love her.” The visits allowed us to bear the grief together; it was our way of coping.

The idea of writing a book together came to me in 2014 after Samarie’s trial. He was sentenced to 28 years in prison for double murder. I thought, the circumstances of my daughter’s death are so extraordinary that I need to find a way to put it in words. Renske was a caring, modest young woman. She and Samarie had dreamed of living together one day. As well as losing her, we had lost them as a couple.

Samarie’s sentence was reduced on appeal to five and a half years. Instead of premeditated murder, he was found to have diminished responsibility for the killing of Renske and partial responsibility for the death of the police officer; the appeal court ruled that he had been in a psychotic state. He has now completed his sentence, and been transferred to psychiatric care.

We continue to visit and support Samarie. It’s not about whether we forgive him. What happened can never be erased, for him or for us. I can understand people thinking it’s unbelievable that we can even look each other in the eye, but this is our way of dealing with it. I’ve never thought it was the wrong decision.

• As told to Gordon Darroch. Do you have an experience to share? Email experience@theguardian.com