[MarcS]

There are good, valid reasons to shoot someone in the back in an act of self defense, especially if they are armed with a firearm. Just because someone turns around does not mean they are retreating. They could very well be booking it to cover to continue shooting at you from a barricaded position. They could very easily turn around and continue firing. You are not out of harms way simply because someone with a projectile weapon is moving away from you.

Do not disengage until all threats are incapacitated, surrendered or no longer in a position to cause you immediate harm.