Why would you use a Taser on a guy when he is cuffed and laying prone face down on the floor with a big ole boot on his neck? Why? Do you think the victim was going to jump up, spin around and kill all 4 cops before they could do anything? The guy just went nuts, that's the only thing I can think of, he just pulled his gun while having some kind of PTSD episode and shot that dude. That's the only rational explanation I can muster, why else would you take any kind of corrective action to someone who is totally harmless?