Whether or not the suspect is armed is irrelevant to the "fleeing felon" standard in the sense that an unarmed violentfelon can be shot in some circumstances. Look up Tennesee V. Gardner which is the US Sup Court case where it comes from. It's a good read and spells it out very plainly. If the violent felon's escape is likely to result in death or serious bodily injury, the Officer, if he or she reasonably concludes that it will, is justified in using lethal force to effect the arrest. Lethal force is the functional equivalant of an arrest under the 4th Amendment. Shooting him in the back, unarmed, while running away can be justified under certain circumstances if the criteria are met. Knife, gun, blunt object, etc...all interchangable. If the Officer reasonably believes the violent fleeing felon meets the standard, the type of weapon or even whether or not the felon is still armed are just factors in his or her lethal force decison but not necessary elements.
I assume the "shoot his legs out" comment was in jest, however, the discharge of a firearm at a person is considered deadly force, regardless of it's intent, which by it's very nature has a strong likelyhood of death or great bodily injury. Shooting to wound is a very bad idea and unless one is a expert in anatomy and an increadible shot, especially during an extremely stressful and volitile life threatening situations, it shouldn't even be considered, and then still never done.