I'll never understand why people say that "might kill them" as opposed to "outright kill them" is better, or preferable. A shooting is plain and simply "to stop the threat", whatever that entails, is justified. You can argue the morality of either later, but even then I still have to stretch my brain to wonder why people find the other preferable.
Further more I find the demands of arguments like this to be pretty comical.
"Police should be crack shots, every time, or step out, and should have the mental fortitude to, under all circumstances, even under fire, use extreme discretion to choose among a innumerable list of options to end the fight, and always pick the best one, when time matters."
It's great and works, if you have the initiative and advantage and the time to sit there and decide how to approach the situation further, if you've got your sights trained on a target for a long period of time, and he's not really doing anything that might distract you from making these decisions, then sure, aim for what you think is best. But I think it's rather rare they might be given that opportunity. And remember that stray bullets from misses are NOT preferable, as they can strike other things you don't want to hit.
So then the best course of action is to "shoot to stop the threat". Also remember that you can't count on ballistics and people to be uniform. One person can take a bullet, and keep truckin', whereas another person might take that same bullet, and the fight is over.
In the instances you provided, where a man was shot in the legs, we don't know what happened when they hit his legs, and there's no way to tell what would happen, either. And, if we're going to go off anecdotal evidence, let me bring up the killing of officer Jeff Moritz, who was shot in the shoulder with a .22 caliber handgun. One of the five bullets ended up in his heart at the end of its travel through his shoulder and down, and killed him. The chance to miss, is high, the chance to strike something critical like a main artery, is also high. The former is harder to mitigate, see above on taking time to aim and decide, the latter can be "solved" by immediate first aid once the fight is over, but it's only preferable in so far that the other guy is still alive, and it really depends on if what's struck is saveable.