Based on what is written in the story it is hard to fault the cop.
He identified himself as the two, Harvey and his “passenger” approached.
While I tend not to believe the cops, who have a sorry history of lying, there is still too little information presented.
Seems to me the local police and DA knows more than us.
Let’s not forget that he’s currently in jail based on this. If it was self defense, there'd probably be no arrest (and I acknowledge this is an assumption, also. But cops don’t arrest cops generally unless it is a very blatant crime).
From the article: Troopers charged Walker on Sunday with second degree murder and manslaughter, and he remains in a Maryland jail on $1 million bond.