I'm loath to be too harsh on the writer of that piece, as he has suffered his own personal tragedy, but the statistics used to back him up are scattershot at best. The headline number of "900 mental health homicides have taken place since 1993" fails to say how that is defined or give it any context. 5 mins research shows it's based on the number of people found to have mental health issues
after committing a murder, with no information on how many were known to have these issues
beforehand, and when you consider that "the authorities should have done something" is the main thrust of the piece, that's just shoddy. The context is that that's in the region of 10-15% of murders.
Worst of all, the campaigning website linked in the piece is only claiming 20-30% of these are preventable (and their basis for that claim is itself a bit dubious) - a statistic that strangely doesn't appear in the Telegraph, presumably as it destroys their attempt to imply all those deaths were someone's fault.
The simple fact is that you are massively more likely to be murdered by friends or family than a random mentally ill person who "ought to have been locked up already."
(And yes, this is now totally off topic, so I'll leave it there)