If the police were on alert for the suspect, they probably already had their guns drawn when they knocked on the door. Then as they heard the door being opened, they would likely aim their own weapons at whoever was opening the door. If the guy was pointing his weapon at them, then they would likely fire before he was able to shoot one of them (since they suspected they very well may be facing the dangerous guy they were after). Now, if the guy was pointing his gun at the floor, then they likely would have told him to "drop your weapon".
It is possible that they shot the guy without provocation.
Honestly, unless more info comes out, I will give the cops the benefit of the doubt. I don't think more will come out, considering the circumstances of the incident though.