Possibilities: 1) Fistfight, can’t stab guy if you’re losing. 2) You’re on the floor, beating continues, MAY fear for life, PERHAPS use knife, gun. 3) But in this case, thief goes behind counter (trespass), beats elderly clerk to floor; this is strong-arm robbery, deadly force should be okay. Ask lawyer about law in your locality.
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The most fundamental right of all, the right to life, affirmed in the Declaration of Independence, necessarily includes the right to defend your life, or it is meaningless.
