Nah. Relying on the stabber's self attestation "oh I felt really threatened" is always stupid. That's how you get situations like Trayvon Martin.
You never know whether someone will attack you. Giving people the right to stab any person who makes them feel threatened is de facto giving people the right to stab people at will.
Indeed. But if somebody were actively attacking you, having not yet inflicted any harm, I suppose any jury would see your self-defense as self-defense. You don't need to wait to be stabbed to fight back.
OTOH if you were to say "he just looked at me just so, and I felt really threatened, so I fought back", it's likely not going to fly.
You never know whether someone will attack you. Giving people the right to stab any person who makes them feel threatened is de facto giving people the right to stab people at will.