There is a report him saying "I got him" after he shot him. Also a report of him kicking him after he shot him.
"I got him" is most probably letting his partner know that all clear Brooks isn't coming back for us... no some triumphant statement about "yeah! I managed to kill a black guy! That's why I always wanted to become a cop!"
Was kicking to ensure he was immobile? Maybe he wanted to make sure the bullet hadn't grazed his arm and Brooks was faking?
Although if the officer has had run ins with this guy before that might be different. But that still wouldn't be bc he was black rather a personal fight between the two.