Earlier quoted context omitted.
You forgot the https://en.wikipedia.org/wiki/Hungerford_massacre in 1987 where 16 people were killed. Both events lead to changes in firearm licensing rules, and in both cases if the existing rules had been correctly enforced, it's doubtful either person would have held a licence. EDIT: And https://en.wikipedia.org/wiki/Cumbria_shootings in 2010 10 dead, 11 injured.
It appears everything was correctly enforced? He went through the appropriate channels - filled in the correct forms, and appeared sane at the time of application. I guess another question would be, why on earth would anyone need _any_ of those guns.
I can understand wanting to collect, or at least try, different types, but who in the world needs an assault rifle?