Fleeing Felon rule...
"A police officer may not seize an unarmed, nondangerous suspect by shooting him dead...however...Where the officer has probable cause to believe that the suspect poses a threat of serious physical harm, either to the officer or to others, it is not constitutionally unreasonable to prevent escape by using deadly force."
— Justice Byron White, Tennessee v. Garner
What also happens is such cases is that the suspect may be in the process of turning his body and the minute amount of time it takes the officers to mentally process what is happening and bringing their sidearms to bear and shoot can make it look potentially questionable. Of course there will always be someone complaining it wasn't necessary to shoot. I say it was completely unnecessary for the suspect to attempt armed robbery.
Based solely on publicly released information it looks like a good shoot.
