We have no idea of this man's needs, why he did it, and what his history is. Therefore, we've no idea how predictable this was, whether breakdown in processes led to this situation, or how much he can understand actions, harm or right and wrong.
Some thoughts though. Some people with learning disabilities who have a history of risk behaviours are regularly out and about in society all the time. That you don't hear of more of these kinds of incidents is because they are well supported both in learning different behaviours, and in being prevented to have the opportunity by carers who are constantly aware of the potential.
In fact you are less likely to suffer from the actions of learning disabled people who are out and about, than you are from non-learning disabled people, regardless of similar history, and as a percentage of each group. Learning disabled people are more likely to have their world restricted due to behaviour, illegal or not, then Dave down the pub who likes throwing his fists at the end of the night.
Yes, preventative measures fail (rarely), but then so does every kind of public protection measure, in whatever area of the law, care, mental health services, health and safety, fire prevention etc. It fails FAR less with people with learning disabilities, than anyone else, not least of all because they rarely have their own voice.
As additional info, my experience in this is flipping massive, the majority of people who due to intellect, neurodiversity, as well as additional mental health needs, may not udnerstand or be able to control their own behaviour and it's consequences, are not a major problem in society. That they are not is testament to the responsibility that professionals, carers and families take on, and the dilligence that they apply to protecting both individuals with disabilities, and the wider public
As always, knee-jerk reactions will always call for more restrictive measures to be taken, and then, when the next Panarama doc comes out showing the consequences of that, they'll ne calls for better care, meanwhile, professionals who actually understand the risk, will take measures that are extremely complex, and, the vast, vast majority of the time, get it spot on.