Could it be the home of a drug dealer? Someone expecting to be invaded.

He a several other officers were chasing a suspect into the house after a sting operation in which the suspect(shooting victim) was seen selling drugs.

It was a legal entry. No warrant needed.

He was the first to make it into the bathroom were he thought he saw the suspect with a gun and shot him. He had reason to believe the suspect had a gun because the other officers told him so.
The suspect was found with a huge bag of dope and the police now think he was trying to flush it.
No suspects gun was found.

This case was thrown out because the prosecutor told the jury that what the other officers told this cop did not matter and to ignore it all.

It was in effect a mistrial.

He will more than likely be charged with something other than first degree murder later. Which he will more than likely win.
He is really only being brought up on charges to keep the black community quiet. And I bet prosecutors are elected in that area so of course the prosecutor wants good relations with the people who will elect him.