Here is my thing about this.
The police are taught that when you fire your weapon, you shoot to kill. If you are trying to take them alive, in which case you feel it is safe to try to take them alive, you use a taser or non lethal weapons. At the point which you fire your weapon, it is always for the chest, to kill. As such, they are taught to make sure the target is dead. In many cases you have adreniline running through the officers body along with, multiple officers. Think of it like this, for those who have played games like Area 51 or Time Crisis, did you stop at one bullet for each target? When I got into the heat of the game it wasn't uncommon for me to shoot two or three on one person by accident.
So factor in their training, which is, if firing, make sure he is dead. When an officer fires, most the time they unload their clip, and if the person is still standing, then reload and keep firing. I believe the standard police officer's weapon carries fifteen rounds. SWAT works in teams of four to six I believe, but I am not sure about that. Five(average SWAT unit) times fifteen is seventy-five bullets. Now that is a side arm. SWAT, correct me on this if you are sure, works with heavier weapons that have larger clips. With everyone firing as fast as they can to make sure someone who might kill someone on their team, it really isn't surprising to have dozens of bullets into a perpetrator.
People get too used to media, movies, books, and TV shows where the cops are, one shot, one kill. But in real life it rarely works that way. Consider a officers worst nightmare, PCP. Often times they will keep coming after an officer even with thirty plus bullets in them. The reality is that for an officer who crosses the line to use his firearm, the most logical action is to do everything in their power to make sure that the person they are shooting at is dead, or at least so close to it that they can't hope to shoot back. On that same note, the claim that the officers thought he fired the weapon, when you are in a high stress situation with the possibility of you, or one of your teammates who are like brothers being shot, while adreniline is flooding your body. It is to be expected to ready incase the worse happened. As such, he could have heard, or seen something else. Say the man shifted the guy to point at one of his buddies, jumped the gun thinking the man was firing.
In short: Cops are taught that when they fire their weapon, they go for the kill, multiple officers firing at a single target until he hits the floor and possibly a few seconds after that results in a large quantity of bullets. If they announced they were with the police upon the raid then if he held a weapon upon their entering in a threatening manner, from my understanding it is not obvious if the safety is off from a distance, then they thought they were defending themselves.
I'm not saying that there weren't mistakes that did, or might have happened.(I don't trust the media on these cases) But I don't believe that some things warrant, the "gasp how could they" response the media gives to any time an officer fires a weapon.