There exist NO forensics in the world that can determine if he was still threatening you or not from the floor.
Not to nitpick, but you can't bank on that. I have been able to determine by means of X-rays alone, that the gunshot patient's version of events is not compatible with the trajectory of the projectile. It all depends on whether your 'observed' position of the guy's body and limbs is compatible with the terminal trajectory that they reconstruct (using chemical analysis, autopsy, radiology and physical examination of the weapon and the cartridge components).
There is a lot of risk involved: you've got to have your ducks all in a row or you may be found out.