The line of fire was short ranged and clear. It was an easy shot to blast the gun from his hand or hit the hand itself.
Amazingly, the 'rules' do not permit such efforts. Effectively, police marksmen are required to use 'a killing shot' only.
Whether saving Moat's life was a desirable outcome or not is a wholly different matter.
But naturally, there are some cases where somebody has simply had a mental breakdown and every possible effort must be made to save their lives.
We only have to consider the tragic events outside Hull on the A63 several years ago to have this point confirmed.