Yes, some overreact. Given the description we have thus far I'm wondering whether this was a "negligent discharge" by the agent. It's easy to have this happen if one has his finger on the trigger when it shouldn't be. The combination of stress and a small inadvertant physical movement could easily account for the weapon being fired.
Further suppord for this being an "ND" is the shot wasn't centered on the head, despite a very short distance from the muzzle to the target. I believe we'll find the agent did not intend to fire. This really goes back to training, which is apparently inadequate, IMHO.
FBI firearms procedures are inadequate by California law enforcement standards.
And what accounts for them leaving the kid handcuffed face down on the ground bleeding to death, refusing to allow medical attention for an hour, until they realized they'd shot an innocent?
This was a clusterf'ck, bigtime.