On first blush it looks unconscionable. I will assume (arguendo, bear with me) this isn't a function of racism. Looking at the reports one of two things happened.
1: The cop committed a willful homicide.
2: The training of BART cops, and the SOP for using TASERS is so poor this was an honest mistake, made within the acceptable guidlines for the use of force.
Looking at the footage, I think it's the latter, and that (in my opinion) is worse.
Digby has been writing, for months; perhaps more than a year, about the increasing use of TASERs when not really called for.
This looks to be a case in which the officers were frustrated by the subject being less than completely compliant to their wishses. I don't know how much work they were having to do to as they tried to cuff him (which, from other reports was being done with flex-cuffs, which are a little harder to use than "bracelet" cuffs).
All the cops got up at once and stepped away as the one draws a weapon and shoots. That's what makes me think it was supposed to be a TASERing. They didn't want to get zapped with conductive energy.
Me, I am not so fond of TASERs, and the more I read about them, the less I like them. They are considered, "harmless", never mind that accounts of being hit with them are of excruciating pain. Of people who say they would rather be pepper sprayed, physically beaten; or even shot, rather than ever be tasered again.
But the, "non-lethal" nature of them (despite the people who die after being TASERed), means police depts don't investigate the use of them the way they do the discharge of a firearm.
When you add in that TASERs are built like handguns you only increase the risk of the possibility they will be grabbed by mistake. I've seen police officers who have their TASER in the off-side, so a cross-body draw is required. That's a step in the right direction, but I've also seem them placed on the strong-side, just a lot further back.
What I've read is BART doesn't have a 1:1 ratio of tasers to cops on duty, which means not all officers carry them. When they do have them they are handed off from one cop to another at shift change.
That's a recipe for disaster. Trust me, weapons need to be familiar to the bearer, esp. if they are to be grabbed in moments of stress. My guess (and it's just a guess) this cop normally has a TASER. He carries it strong-side, behind his 9mm. He wasn't carrying it that night, and forgot that fact.
Pressing the armchair analysis, if he hasn't used a TASER before, he's seen it done, more than once. The shocked look on his face, after the shot goes off, makes me pretty sure he didn't expect what happened. In the cell-phone video he seems to be trying to explain the gunshot to his partners, before they start looking after the victim.
Which means, sadly, this is several different tragedies, all running in parallel.