Here's a possibility
Five shots in the face and then a stomp on the head sounds rather more personal than a rival gang member taking out a random member of the HA.
It could be over a woman, and a love rival put the five shots in the face as the victim was the prettier one of the two, and the stomp on the head was because he had scored with the said woman whereas the killer hadn't
Him being a member of the HA was just coincidental
