Even though Brown was about 30' away, the cop likely only had seconds [see theory in quote below from another source] to stop an 18 year old, 6' plus guy before he would reach the already injured police officer and could finish him off.
"30 yard dash by a 6 footer-plus 18 year old sprinting?
Count with me
One-one thousand, two-one thousand, three-one thousand, four-one
. Wilson is dead man.
Except for the last ditch effort at Brown's forehead, two shots at once. One hit to right of eye instead which is consistent with Wilson's right drag (like golf, people have a tendency in their technique to favor one side) and the last shot in succession struck this time home but top of head as Brown falls.