Whether they walk away out of fear or higher principals, the important point is they walked away, thus avoiding a fight...
I pretty much agree with everything you said in your post except for the above statement. It seems to me the reason behind why you do something is just as important, if not more so, as the action itself.
To respond out of fear is to allow your emotions to control you. Whether you back down out of fear of being hurt or attack out of the fear of being seen as a coward, you have still reacted based on your emotions. Now, I'm discounting emotions, but what separate humans from the animals is the ability to act out of our intellect. You like you stated in your post.
I agree it is better to walk away out of fear than to attack out of fear. But good aikido is about being centered keeping your one point. If you are centered and choose to walk away from a fight point for you, but if for some reason you choose to fight do it from a centered position. I believe that's what you where saying when you said, "Regardless of my own fear...I've simply adopted a relaxed, ready position and stared silently at my antagonist.
Just my two cents. Again, you pretty much said what I wanted to say in your post.
Peace and Blessings.