Based upon the scant information presented in the article, any judgment being passed now is woefully premature.
Critical questions remaining to be answered from other sources:
1. Did the officers announce that they were police?
2. Did the officers provide a reasonable amount of time for the door to be answered?
3. Who opened the door?
4. Did the homeowner have the gun in a holster or in his hand?
5. Did the homeowner point the gun at the officers?
6. Did the homeowner say anything leading the officers to believe that they were in danger?
7. Did the homeowner resist the execution of the warrant?
The officers were trying to arrest a person who already showed no compunction against shooting at them. They were understandably in a hyper-ready state. Unless we learn more details, reasonable judgments could range from, "These cops wanted to exact their revenge, went to this home fully intending to get into a shoot-out, were on a hair-trigger, and used the slightest (unjustifiable) excuse to open fire," to, "These cops, who knew that they were serving a very dangerous warrant and knew that the suspect was predisposed to shooting officers, announced themselves as officers, found an armed man (gun in hand) at the door when he opened it, and reacted to his verbal resistance, combined with his dangerous handling of his weapon, reasonably believing that he was about to stop the execution of the warrant by using deadly force."
Anyone who claims to know which of the above (or any other scenario) happened based on the scant details provided by the article is just blowing so much smoke.
My current inclination is to think that the officers were probably too ready to mix it up, but, like I have not yet heard in this thread, I don't know.