This was a tragedy and missed opportunity, but this was not "terrorism" in the sense being used. This was a seriously dangerous, violent and mentally ill person.
The focus on how Prevent failed is a real problem because the Prevent strategy is there to stop people being radicalised, not to police society for violent individuals.
The focus here should really be on the mental health system & social services, and how an extremely dangerous person who was excluded from school for violence, went to a special school and could only be dealt with as a home schooled student for staff and student safety, and for whom their family raised concerns. This guy was not radicalised, he was not a "terrorist", he was someone who was clearly flagged as very dangerous as a child and failed to act.
There have been dangerous people before, and there are laws and structures to detain extremely dangerous people for public safety. That's where everything failed - yet again Social Services, Police, Health and Education. It's a pattern seen over and over again with public agencies not working together, often because they're under resourced and stretched as it is.