My take: It does not matter either way. The black man was retreating and was ZERO threat. I see it as premeditated murder.
If it is determined that the officer was reasonable in assuming that Scott still had possession of the taser gun then deadly force could have easily been justified within the three second turn of events. If the officer was tasered before the video, or, if he was in fear that the taser would be used against him to disable him, then he might be found not guilty of murder or manslaughter. Law enforcement are thought to utilize deadly force in an event that incapacitates them enough that a perpetrator can gain access to the weapons on his belt. I was taught that in the military. Especially when the perpetrator has demonstrated a willingness to do harm with the weapons on the belt. Furthermore, a taser can be used over and over again until the battery has run dry, and/or, incapacitate the officer and follow up with the beating. This could easily be interpreted as a means to do serious bodily harm which is also a justification of deadly force.