It is of course possible for it to be determined in advance that a person is insane and not fit to stand trial, Teribus, but it is also quite possible in a trial for the grounds of a successful defence to be that the defendant is not guilty by reason of insanity.

The fact that this man was killed was not a case of "summary justice". He wasn't being punished for his actions, he was being prevented from causing further harm. If the NHS had succeeded in their efforts to prevent him dying, that would have been a better outcome.