An "immensely dangerous" prolific offender who brutally attacked and killed a young woman just two weeks after being released from prison was a "time bomb" and the woman's death was a terrible thing waiting to happen, according to a forensic psychologist.
Professor Robert Snowden, a lecturer at Cardiff University, said even a "cursory" pre-release risk assessment should have revealed that despite Matthew Williams' apparent mental stability while in custody, the chances of him deteriorating as soon as he was released were "massive".
Cerys Yemm, 22, died from her injuries after Williams attacked her using shards of a broken cereal bowl in his room at the Sirhowy Arms Hotel in Argoed, South Wales, on November 6 2014.
Williams was Tasered by police and died at the scene after suffering a cardiac arrest.
Prof Snowden studied the Healthcare Inspectorate Wales report on the deaths and looked at evidence from the inquest at Gwent Coroner's Court which ended today.
He did a risk assessment based on what was known before Williams, 34, was released from HMP Parc, Bridgend, on October 23, 2014.
He said the results of his HCR-20 assessment - "the gold standard of risk assessment for violence to others" - showed it was "obvious" this was "a terrible thing waiting to happen" and pointed to Williams' history of violence and his "strong anti-social or anti-authoritarian streak" as important factors.
The inquest heard a probation risk assessment compiled on October 10, 2014, two weeks before Williams was released, classed him as a "medium risk" of serious harm to the public.
The probation officer who carried out the assessment said there was nothing in Williams' past that gave any indication he might commit an offence such as the one on Miss Yemm.
Prof Snowden said he would have classed Williams as a "very high" risk of harm to the public.
He said: "The thing to really notice is first of all he uses weapons, which means that the violence could be fatal and that he attacks just about everybody ... this is a person who is immensely dangerous."
Prof Snowden said Williams' non-compliance with medication and the efforts of people to help him, plus his "massive amount of drug use from an early age", were other risk factors.
Williams was diagnosed with schizophrenia in 2004, though the diagnosis was subsequently questioned, and the HIW report concluded it was "more likely that he was experiencing drug-induced psychotic episodes".
Prof Snowden pointed to confusion over diagnosis as a problem in identifying the risk Williams posed.
He said: "They were busy treating his possible disorder and wondering what it was when they should have really been looking at his attitudes and his violence."
Prof Snowden added: "The problem they seem to have is that he had served his time and therefore had to be released, and was absolutely refusing any sort of help, so they seem to have got his mental health stable for a moment while he was in prison.
"I think even a cursory risk assessment would have realised that the chances of him deteriorating as soon as he was released were massive, that he would take drugs again and this would affect his mental health again, and he would be back to his most dangerous within days.
"So, what really happened is that the drugs pour fuel onto that fire. He would be a dangerous person anyway but with this massive amount of drugs inside him, he was a time bomb."
The inquest heard even if Williams had been considered a high or very high risk, probation could only offer services on a voluntary basis because he had served his whole sentence.
Prof Snowden added: "Good risk assessment is at the heart of being able to manage someone like this and I say this both for his safety and clearly for the public's safety.
"It looks to me that the risk assessment done was not very thorough."
The inquest heard Williams sent a number of letters to his ex-partner from prison, making threats against her, his family and police officers, and that these letters were handed to police in April 2014.
Police had been looking into the possibility of pursuing a charge against Williams over the threats in the original letters, though his former partner had not wanted to get involved, the jury heard.
Of the threatening letters, Prof Snowden said it was "amazing that those weren't acted on" as they would have provided an opportunity for Williams' to have been detained.
Dr Jane Monckton Smith, a lecturer in criminology at the University of Gloucestershire, said the threats from prison were "a high-risk marker" and showed Williams was feeling wronged, resentful and vengeful and that Miss Yemm was "in the wrong place at the wrong time when she came across him".
"There are elements of stalking mentality here too," she said.
"If he has a history of domestic abuse, this links strongly to control issues and a sense of entitlement. This is a flag for his problems with women.
"The high level of violence directed at (Cerys') head and face is about rage and hate."