I've been through a couple different schools that teach shooting and both of them taught controlled pair center of mass then track to head. Head shot if the threat continues. I doubt that would help if vision was blurred due to injury.
Another thing I was taught at both places was that there is absolutely nothing wrong in and of itself in shooting a person in the back. If you have any reason to believe the threat still exists (the person is going to shoot while running, the person is running for cover to shoot back, etc.), you pop the guy while you can.
Finally as others have pointed out, if you start ripping into the trigger (a sign of poor training perhaps, but who am I to judge), there is no reason to believe the shooter realized the final shoots were going into the assailant's back.
As for being sued, that is a constant. The stories I've read that ended in being sued are nothing short of incredible. Frankly I'd love to see lawyers personally liable for bringing frivolous suits to court since by and large they do it knowing that the respondent will settle for a few thousand rather than defend themselves in court even if there is absolutely 100% chance of winning. In a lot of cases, it is an insurance covered incident. Probably not in this case though.