Is there a duty to monitor mental health patients? Peter Wrighton/Alexander Palmer

[ShowMeTheElf]

According to the newspapers, 83Yo dog walker Peter Wrighton was repeatedly stabbed in the neck so badly that initially it was considered an animal attack.
There was no obvious suspect, but Alexander Palmer was charged and is now on trial because a psychologist from his mental health team from when he was in the army back in 2015 recognised the crime as exactly what Palmer had fantasized . If the psychologist had not come forward no-one would have known because there was no link between Palmer and Wrighton at all.
Is there no system in place for those with significant mental health issues and violent tendencies to be monitored? If Palmer's mental health issues (voices, violent ideation etc) was significant enough to get him invalided out of the armed forces, was no-one keeping an eye on him? I don't know how the system works but surely this should not have happened?

Verdict now in. Life with a minimum 28 years to serve. So they believe him to be sane in spite of the voices in his head.
Original question though: should someone with such violent fantasies and a mental illness be monitored in some way? How does it work? Does anyone know?
Thanks

Hospitals can detain and treat people who have a serious mental health problem and who are a risk to the safety of themselves or others. There are also more limited community based powers. It does sound like there were failings in this case.

Which mental illness were they diagnosed with?

Agree there's no mention of what if any treatable psychiatric condition he had. People with personality disorders often claim to hear voices but many aren't treatable. Prisons are full of people with personality disorders many of whom have very little empathy. Drugs don't change this.
Also some people are more bad than mad. Violent ideation isn't a sign of psychiatric illness but a sign of being dangerous.

He was invalided out of the Paras because of mental health issues. I'm not sure that his diagnosis matters really: he is a combat trained violent man clearly disturbed who has shared specific violent fantasies (significant enough that 3-4 years later a psychologist recognised them from a news report) and has 'voices in his head'. What I'm trying to understand is whether or not he would have been monitored in some way or if someone with such a condition would be discharged from the army and left under the care of the community/a GP or what?

No they aren't able to do this - hopefully he would have been asked at the time if these where thoughts or did he intend to do it - he would hopefully have been asked if he carried knives -he may or may not have had a forensic assessment - to look closer at his risk indicators. - he may not have engaged in therapy - mental health services operate on a recovery model - meaning patients no longer stay in the system for life - it's has its advantages and disadvantages.
There would hopefully have been an enquiry into this death and I'm sure the mental health team would have been asked for information.
The murderer has been sent to prison. He was not deemed to be mentally unwell when he did it.
It's no comfort for the dead mans family.