And, since he was shot outside, arguments of friendly fire need to die. Furthermore, reports from witnesses state he was the lone shooter and was very methodical about it, sometimes using a second round to ensure he actually killed someone. That was what he was doing when our heroine ended his killing spree.
And a last observation from news reports and experience, he chose a location with people lined up and grouped. That made his desire to maximize his body count simple. Everyone was lined up like ducks in a shooting gallery.