I just saw this linked via Yahoo and was going to post it. You would think that a seasoned FBI agent would be able to subdue and restrain someone without using lethal force.
You know what they say: From the head down...
I read early on that the guy was a MMA competitior on a local basis.
He may have waited until the agents guard was down in what appeared to be a friendly interview and then just uncorked on him. It would be easy to be on the downhill side of an encounter like that and have no other choice that to use his weapon. AKA-George Zimmerman.