The argument was that it was wrong to shoot them at all. Forget about where the bullet goes.
Right, I was responding to your assertion that you would shoot them in the leg if they had a knife, but the point is that a) that's pretty difficult for all but the most accomplished close quarters combat veteran to do, b) it's not going to be possible or helpful in an awful lot of situations, and c) there's not really any such thing as a non-lethal gun shot - shooting them in the leg gives you a reasonable chance of hitting their femoral artery.
I honestly don't have a problem with a cop shooting to kill if an assailant is coming at them with a deadly weapon, including a knife. Any bouncer will tell you that they're far more worried about knives than pretty much anything else, no matter how much bigger and stronger and proficient you are than your assailant there's still a very high possibility you're getting opened up in the process of overpowering and disarming them. If they're not armed, that's why you have tasers and batons and training.