I really can't offer many answers but I do know he needs to receive a whole life term and the Met need to be investigated as to how he became an Officer with, presumably being a Diplomatic Protection Officer, access to firearms.
Edit: He just got a whole life order. At least the judiciary did...
I have gone through SC clearance and that is fairly invasive but the Developed Vetting (DV) he would have gone through would probably have found out things about him he didn't know about himself. Plus presumably there is regular psychiatric evaluations for Policemen who can carry firearms...