The police use their stun guns on people who are already on the ground and offering no threat all the time. It's no wonder that one of them would get confused and just start shooting people in the same position. They have no sense of what constitutes a real threat anymore.
See, the problem isn't the form of gun they use, a stun gun or one with bullets. It's that they use any gun on people who are already down. They do not have a right to inflict pain or kill people who are already in custody, for any reason.
Digby might be putting the most charitable construction on what was done here: "They have no sense of what constitutes a real threat anymore." It could also be about abuse of power, sadism, punishment, racism.
This happens in America. And as economic times get tougher, as more people slip out of the middle class comfort zone into poverty, expect similar incidents to happen more frequently.