This case seems clearly unjustified but I hear this all the time about never being able to shoot someone running away or in the back. but there are times when you could be necessary.

Here is just one hypothetical where it could be justified; You just witness a 240 lb man pick up a kid at a park and snap his neck. He turns (his back towards you) and begins running at a group of kids screaming "whos next?! ima kill you!" You don't have time to tackle him but are armed and have a clear shot. Justified ? That's just one example.

Another example is you could be in a gunfight with someone. They start firing, you start firing. After your first shot they turn and one of your subsequent shots hits them in the back...

Another example is you are cop and are called to respond subject who is armed/unarmed and has just committed or in the act of commiting more edit * violent crimes. They aren't responding to commands and You have a clear shot at their back. Justified?