St Johns, would the law accept it as justified? Depends on what kind of day the prosecutor is having, and what the judge had for lunch.
According to the statutes here in my state, it doesn't just need to be my life in danger. If I have a reasonable belief that one of my family members is in danger of grevious bodily harm I can use deadly force. If a man was threatening my wife with a knife and had the means and ability to harm her, but I was out of the danger range, I could still shoot the guy with the knife because I was in fear for her life.
As for the car speeding off into traffic with a dead driver, like I said, it depends on if I have a good shot. The relative speed of the car and where it is heading are part of what makes it a good shot or not. If it was cruising along at forty miles an hour and heading up the freeway onramp, then no I wouldn't shoot. Pulling away from me in the parking lot with a kidnapped child in the back seat is a different story however.
I would rather my nephew take the risk of a low speed car wreck in a parking lot (he is buckled up I assume?) rather than let somebody who might be John Wayne Gacy reincarnate drive off with him. A reasonable person would not automatically assume that the bad guy is going to pull over and let the kid out.
As for shooting a fleeing felon in the back, and getting prosecuted for it, you are shooting a felon in the act of committing a violent dangerous crime (kidnapping), not a fleeing felon. Now if the bad guy jumped out of the car and started to run away, and then I shot him in the back I could potentially expect to be prosecuted for shooting somebody when no one's life was in danger.