If you were in fear for your life before, and now they drop their weapon and are advancing on you, they're not advancing because they want to give you a hug. A reasonable person would believe that the BG is advancing to try to get your weapon and kill you with it.
"Don't come any closer or I shoot!"
[BG moves closer despite warning]
Pretty easy to articulate you feared for your life. He already showed means, opportunity, and intent to hurt/kill you. Then, after being warned that he would be shot if he came closer, he came closer anyway. Someone not afraid of being shot, who has already shown an intent to hurt/kill you is a deadly adversary. If he got close enough, he would kill you. VERY reasonable fear for your life right there.
At my agency, such a shooting would be deemed justified under our use of force continuum (and would be within policy for Agent involved shootings too!)
Last edited by blk69stang; 11-20-2012 at 00:43..