The absence of a decision to carry is not the same as a decision not to carry.
There's a lot of history behind this. When establishing the world's first modern police force from the 1840s, Peel and his successors were always insistent on the distinction from the armed police forces of the European continent. Those were organised along military lines - indeed were often part of the formal military structure - and were unmistakably instruments of State control. The British police, by contrast, were to be a civilian force, accountable not to the state but to the local community through 'Watch committees'. The absence of arms was in part a symbol of this civilian status. The police could always call on the armed services - the local Yeomanry - for support when necessary, which it rarely was.
Despite all the fundamental social changes since that time, something of that ethos still survives, particularly in the attitude of the police themselves to the armed/unarmed dilemma. And it has to be said that armed attacks on police officers in Britain are still extremely rare. This was only the fourth such fatality in the last seven years.
Death is such an abstract construct when it happens to someone else.