Christopher Laskaris, who suffered from Autism, was murdered by Philip Craig who then returned to steal from his flat days later - and even rifled through his pockets as he lay dead on the floor.
Craig, who was sentenced to life in prison after a trial, had a long history of violent offences and was out on licence when he stabbed Mr Laskaris to death on November 4, 2016.
Items including a television, mobile phone and electric guitars were stolen from the property in Leeds, West Yorks.
A post-mortem examination found the cause of death was a single stab wound to the chest.
Fiona Laskaris, his heartbroken mother, today (Tues) told the inquest Christopher suffered from an Autistic Spectrum Disorder which was never formally diagnosed and was only ever informally recognised by doctors.
Weeks before his death, Mr Laskaris's door had been broken down by police officers who were concerned for his safety, the inquest heard.
Officers passing his home had heard shouting and saw a coffee jar thrown through the window of his ground floor flat when they intervened.
Mrs Laskaris said her son was undergoing a mental health crisis during the incident and was taken to a police station for his own safety.
When Christopher was released, his flat was boarded up and she told the court she believes it was then that he met Craig who helped him move the board.
The inquest heard that phone records indicated Craig had called Mr Laskaris on October 13, the day the door was broken down.
Mrs Laskaris told Wakefield Coroner's Court: "I don't think he single-handedly and without tools could have removed that board.
"I'm very concerned how my son was left in the position that a murderer could have access to his flat.
"This man met Christopher because his flat was left open.
"Christopher was frightened to knock on a neighbour's door if a package was left for him.
"He was too shy to go out and socialise, he would never have met Philip Craig in the community.
"I just ask where we are as a country that this could happen.
"He was such a lovely young man with so much to offer, but he couldn't get that little bit of help that he needed to function in this world.
"It was a living nightmare and continues to be a living nightmare three years later because we just can't get answers."
Mr Laskaris's body was discovered on November 17, 2016, when his mother, who lived in Surrey, called police and raised concerns over his welfare.
Mrs Laskaris told the court she had struggled to get an official diagnosis of Autism throughout his life and his intelligence belied his lack of life skills.
Her son earned a scholarship to Charterhouse School in Surrey as a youngster, but could not cope with the pressures of "every day activities like going to school", she added.
She said he would bang his head against the car door and try to grab the steering wheel as she drove him to school.
He had moved out of the family home and was living alone more than 200 miles away in Leeds, West Yorks.
He had initially stayed with his aunt in the city, but she struggled to cope with the care he needed and he was eventually given a council flat in the Hyde Park area after being referred to mental health services.
Mrs Laskaris said her son "just could not function" well enough to live independently, but the lack of diagnosis meant any concerns she raised to the authorities were not acted upon and her son was left without the specialist support he needed.
She added: "When that knock on the door from the policeman came on November 17, I had been waiting for years for it.
"The last time I saw him in Leeds was April 2015, his flat was in the most terrible state and it was obvious things weren't working out for him."
The inquest, expected to last two days, continues.