I dont think a taser would be very useful in such a situation, they're better employed against stationary targets. By the time Brown was close enough to be tasered he would have been running full blast at you and probably ready to take evasive measures to dodge the 'shot'. Even with bullets Brown got close to the cop, if he was charging me I'd drop the taser after my shot and ready myself for a collision I can use to my advantage.
Of course a taser is not ideal in all situations, but if you have some hand-to-hand combat training, you have alternatives to killing him. Starting with a stun gun, or some other method of incapacitating him, in addition to the actual combat skills themselves.
I never said the average cop cannot be trained to such standards, but most are no where near such standards currently, But we expect them to have such standards. If most cops do not have such training, then why do we put them out there? Wouldn't it be better to have cops not engage at all? Or do we simply condemn the cop because we believe the cop could have done better, no matter the situation?
We condemn the cops in part because, rather than come out and say, "We aren't trained for this, please give us better training," they turtle under the protection of their unions, and nothing ever changes. They simply aren't trying. As you note, the status quo isn't working. But there is no impetus for change that would save lives. There are of course exceptions to this, as plenty of police departments have made significant changes. But there are many more that haven't.
If there was a widespread, honest effort being made by police departments to train officers in threat assessment and de-escalation, and sincere attempts to hold officers accountable when they breach protocol and a civilian ends up dead, then there wouldn't be a problem. Nobody is demanding perfection, because perfection is impossible. Likewise, nobody is arguing for unreasonably strict standards.
Also, they're human beings. Generally, when a human being escalates a fist fight into a shooting, we think poorly of that person. We call it murder. We mourn for the life cut short. Setting aside the badge, it's just garbage human behavior to shoot an unarmed person.