Thing is.. you don't need a manual to tell you how to use a car to kill people... you just drive at them
Not entirely true. The pedestrian safety measures in a modern car like the ix40 as used in Westminster will render it unusable after the first few hits and/or fail to produce anything like the damage you are expecting. He hit 40+ people with the desire to kill and failed to kill all but two of them.
So you actually need a manual to chose the right car and/or exactly how to mow people down with it if you want to kill people, not put a few in hospital.
That manual most likely already exist. It is also clear that this gentleman did not have access to it. He would have chosen a different vehicle and driven it differently.
His case will be used for all it is worth by the Government to push repressive legislation despite the fact that he neither operated under instructions, nor had access to them via an encrypted channel. Rudd already started doing it.