So let's assume, arguendo as the lawyers say, the Johannes Mehserle is telling the truth, that he thought he had drawn his taser instead of his handgun when he shot Oscar Grant in the back. I'm not saying I believe him, but suppose that's true.
It's still an excellent argument against giving tasers to the SF cops.
So is this.
I don't think anyone, even the still-clueless BART police, would argue that Oscar Grant had created a situation that justified the use of lethal force. He was unarmed, not an imminent threat to the life of a cop or a bystander. But a taser is just so convenient; you can zap someone who is just a little unruly. It's a weapon that's just too easy to justify.
If Mehserle didn't have a taser, Grant might still be alive.
And I don't care how much training you give the cops: As we saw with the BART cop zapping the fare evader, give them a weapon and they'll use it.