The widow of an academic stabbed to death by a stranger outside their home in north London has demanded to know why her husband’s attacker, who had a mental illness, was “armed and at liberty” after previous charges against him were dropped.

Nadja Ensink-Teich sobbed as an inquest was told her husband shouted “help me, help me” as he was stabbed multiple times.

Jeroen Ensink, 41, a senior lecturer at the London School of Hygiene and Tropical Medicine, was “stabbed by somebody he didn’t know when he was posting some cards announcing the arrival of his first baby”, the coroner, Mary Hassell, told an inquest jury at St Pancras coroner’s court. “He died in the street.”

Ensink, who died on 29 December 2015, had been stabbed multiple times in the chest and thigh. He had defensive wounds on his hands where he had tried to wrest the blade from his attacker, the jury heard.

Femi Nandap, then 23, a Nigerian student who had a severe psychotic illness, was convicted of manslaughter on the grounds of diminished responsibility and in October 2016 he was sentenced to an indefinite hospital order in Broadmoor.

On the day of the killing, Maria Hegarty, an off-duty special police constable, heard shouts and saw one man motionless on the ground and another standing over him holding a large kitchen knife “covered in blood”, she said in a statement to the inquest.

“I shouted: ‘Police, police, get away from him.’” The man with the knife seemed calm, Hegarty said. The man on the floor was making gurgling noises “and wasn’t breathing properly”. Nandap, still holding the knife, walked away, then back again, very calmly.

“He said: ‘Leave him. He’s dead anyway,’” her statement said. Hegarty started chest compressions, but Nandap said to her: “He’s dead now, he’s dead.”

He was arrested nearby.

In May 2015, witnesses called 999 to report Nandap for acting strangely in the street while armed with a knife. He was arrested at his sister’s flat in Belsize Park, north London, after a violent scuffle with police. A knife with a 19cm blade was found on the living room windowsill, the jury heard.

Charges of possessing an offensive weapon and assaulting an officer were dropped by the Crown Prosecution Service (CPS) for lack of evidence six days before Nandap killed Ensink.

Officers admitted that during the first incident they should have created a Merlin report, which is used by police to identify vulnerable people and those with mental health issues, and highlight them to other agencies.

During the first arrest, Nandap shouted: “They are coming to get me, they are going to kill me.” He was muttering incoherently, had punched police officers and tried to bite the nose of Adam Wellings, the arresting officer.

Wellings told the inquest he did not initially identify this behaviour as being a mental illness, and did not create a Merlin report. The coroner asked: “Do you think you should have created [one]?” Wellings replied: “With hindsight, yes.”

He said it “didn’t scream out at me that he had mental health issues”. It was only later the officer learned Nandap’s sister had voiced her concerns over his mental health. He said he liked to believe, though could not remember, that he would have mentioned this fact to the custody sergeant.

Wellings said he was “a bit confused” and “disappointed” when he learned the CPS had dropped the charges against Nandap. The CPS later wrote to him saying a review had ruled the decision to drop charges was “incorrect”, Wellings said.

Stephen McDonagh, another arresting officer, said: “I had thought he may have been suffering a mental health crisis.” But afterwards he was controlled, he seemed more calm and rational. Asked by the coroner if he had passed his concern on to the custody sergeant, McDonagh replied: “I am sure that I would have, but specifically I can’t remember.”

He said he had not created a Merlin report because at the time he did not think it necessary. “I see, with hindsight, I think we should have,” he said.

Ensink-Teich had posed a series of questions to the inquest, the coroner said. Among them, she had asked: “How could he be armed with a knife and be at liberty on the day he killed my husband?”

Nandap regularly smoked cannabis and believed he was a messiah and “on the brink of the ultimate truth”, the inquest heard. He had dropped out of his course at Soas University of London. He had not received treatment for mental illness in the UK.

The day after his arrest in May 2015, his family booked him on a plane to Nigeria. He was said to have received mental health treatment at a clinic there before returning to the UK in October 2015, the inquest heard.

At his trial, Nandap was said to have been diagnosed with a severe psychotic illness and was said to be having auditory hallucinations and paranoid delusions, the jury was told.

Ensink, from Zwolle in the Netherlands, was killed 11 days after the birth of his daughter, Fleur.

The inquest continues and is due to last for three weeks.