The problem is that you don't know if he was quick to shoot or not. You're making an assumption based on the fact that he didn't catch him after chasing him down. None of us has facts on whether or not the officer attempted to chase the suspect down on foot first, details about where the suspect was headed (if headed towards a crowded area, area where detaining the suspect becomes impractical or unsafe, etc.). Shooting may have been the last resort, but you seem to be assuming that it wasn't.
Let me clarify what I mean by saying "detaining the suspect becomes impractical or unsafe" because I know how people are going to take that. If the suspect is in an area where there are lots of people, bedroom windows, or dangerous items (pretty much anything involving gasoline, chemicals, electricity, etc), then it becomes unsafe to discharge your firearm under any circumstances. A missed shot could lead to a dead civilian or multiple victims. You have to look at what's between you and your target, what's to the left and right of your target, and what's behind your target. If any of those are things that you can't shoot, then you shouldn't be discharging your firearm. If you can't shoot, but the perp will be in a place where he will be able to discharge his firearm at you or a civilian, then you need to stop him before he gets to that point. Since we've established that the suspect was believed to have been part of a driveby shooting incident earlier that same day, then it is reasonable to believe that the suspect is armed and dangerous, meeting the above qualifications as soon as he runs towards an area where lots of civilians are present.
