Perhaps it was easier for you to realize than you think. I have always expected that I would respond in a similar manner to your own if I or a loved one were in mortal danger. Can that self-knowledge be a solely conscious thing, or is it of two parts, or more? One part instinct, the genetic experience of every single ancestor before you roled into your physiology; and one part consciousness, your mental capacity to observe possible events in a possible future, which must also contend with social pressures.
My trouble now, having studied aikido for just 5 months, is the realization of how little I know (re: technique). Combine that with becoming more and more aware of the nature of crime/violence in the world, and my question becomes could
I defend myself in such a situation. It's more a motivation to train and reflect, than it is frightening.
Ever heard of Joseph Conrad? Wrote Youth, and Heart of Darkness, and Lord Jim, and the like. Yea, I know, not japanese in the slightest, but there are elements of Budo in the mindset often put forth in his writing and characters. More specifically his work often focuses on what people would/could do in the most dire situations.
I bet a lot of people here have asked this question of themselves, and the more experienced might even regret what they'd be capable of, but still do it, because it's you or them. My favorite example of this feeling is in King Henry the 5th of England's speech at the gates before the city of Harfleur, France,
in Henry V, by Shaklespeare.
(editted to say, no, I'm not an english major)