There is always the chance the guy may not have a knife. In a courtroom a one on one confrontation between two men is hardly ever looked at as a deadly threat. True, the situation in Ferguson or Zimmerman was stopping a lethal threat, but in most cases you will be tried and likely prosecuted for manslaughter, at the very least, if the guy you shoot is unarmed. It's damned if you do, damned if you don't...You must be certain at all costs, at least in the eyes of the court. I don't disagree with your reasons or resolve, but the court probably will. If I am alone I will use more discretion. If my son or family are in harm's way I'll take my chances.
Agreed. Without obvious disparity of force it would be very difficult to justify.
Ayoob (as for example the M. Brown case) demonstrated that if there is any gun present that the very act of someone attempting to take it can be considered a lethal threat.