I know several people who have been dangerous in the past due to psychosis, they are totally different people now they are well. I have a family member who has schizophrenia, he was quite poorly a few years ago, I saw him change from a lovely young man to a danger to himself and others, he was hospitalised (sectioned) several times but is now well and hasn’t had a relapse for several years, it’s pretty easy to spot when he is having a relapse and the mental health team are quick at responding and keeping him and others safe, he may never have another relapse. I work with people with schizophrenia and one of these people will never recover (his illness does not respond to medication), he could never be trusted but his case is pretty rare, most cases are treatable with medication and many fully recover.
When someone is suffering psychosis they don’t really have any control over their mind, they are paranoid, they hear voices which are very real to them and of course this can be very dangerous.
If this lad was so unwell that he needed 2 carers, if he was as unwell as people describe (dangerous, doesn’t take no for and answer) then he probably should have been in a secure unit to stop something like this happening, sadly the way things work is someone has to physically harm someone before they can be put into a secure unit.