Very few people understand how your mind reacts to intense situations. Adrenaline destroys your ability to think and perform delicate tasks, like aiming a gun.
Even a revolver with a long, heavy trigger pull can be emptied very quickly, easily within the timespan required for an adrenaline-addled brain to come to the conclusion that the bad guy is no longer a threat.
Given the details in this article, it isn't hard to believe that the cop is entirely justified. People looking for help generally knock, ring the doorbell, or scream loudly, not try to quietly break and enter carrying a flagpole.
I've seen belligerent drunks keep punching even with multiple broken bones in their hands and out-of-their-gourd stoners get up and walk on broken legs. Short of a direct hit on the spinal column or brain stem, humans can absorb a remarkable amount of damage and keep moving, at least for a little while.
In this regard the 9mm parabellum, especially in 115-grain ball loading, is awful for police work. The slugs have a tendancy to go right through soft tissue, and keep going. Police shooting reports are replete with accounts of people shot with 9mm, in many cases fatally, who still managed to run hundreds of yards before succumbing to their wounds.
Bullets in the arms and legs do not necessarily stop a fight. There is a lot of stuff in the chest cavity, front and back, that can be penetrated and not be life threatening. A 9mm is not a very powerful cartridge. I am hardly surprised that he kept on shooting under the circumstances.