After reading the entire police report it appears to me that trooper brown just showed up and was intent on shooting the guy from the get-go. There was no information in there to indicate that he needed to use deadly force at that time. Dont get me wrong, I'd rather see the guy get killed rather than pay to lock him up for life- thats just a waste of space and resources. But at the same time if it were a civilian that shot and killed the suspect for a reason like knowing he just killed someone then im sure you or I would be prosecuted.
He still had the gun in his hand... If you were a civilian and knew that he had just killed someone and he still had the gun in his hand, you could articulate that a reasonable man would think he would still be an imminent deadly threat...
Also, realize that police officer's use of deadly force often follows different laws than civilian use of deadly force. Under Maine statute a police officer is authorized to use deadly force:
B. To effect an arrest or prevent the escape from arrest of a person when the law enforcement officer reasonably believes that the person has committed a crime involving the use or threatened use of deadly force, is using a dangerous weapon in attempting to escape or otherwise indicates that the person is likely to endanger seriously human life or to inflict serious bodily injury unless apprehended without delay; and(1) The law enforcement officer has made reasonable efforts to advise the person that the officer is a law enforcement officer attempting to effect an arrest or prevent the escape from arrest and the officer has reasonable grounds to believe that the person is aware of this advice; or
(2) The law enforcement officer reasonably believes that the person to be arrested otherwise knows that the officer is a law enforcement officer attempting to effect an arrest or prevent the escape from arrest.
This officer made himself aware, told the man to drop the gun, and he did not drop it. He reasonably knew that the person committed a crime involving deadly force. To me, it's pretty black and white according to what the law is.
If it was a civilian, perhaps the ruling would have been different, but I don't think so considering the suspect was still holding the firearm. You don't have to wait until the gun is pointed at your chest before pulling the trigger. You just have to have a reasonable belief that the person is going to use it.