If someone points a weapon at you that is designed to kill people, I would have defended myself. It's unfortunate, yes. I'd say the only thing the cops could have done differently would have been to enter quietly, however the cops did say they announced their presence as they entered. But the question Is did they respond correctly. And the answer is yes. When someone aims a gun at you, it is reasonable to aim one back. Of course they could have ran away, but you can't really run away from a bullet.
Here's an idea: RETREAT!
A million other things they could have and should have done. no excuse.