How to handle chance encounter with ex?
Background:
Twelve years ago, I ended a non-physical ten year relationship with a high school friend due to the large discrepancy between my feelings for her and her feelings for me which I considered had become an unhealthy situation for me. I ended it in a manner that at the time I felt was most appropriate given the overall depth of our relationship, in-person verbally expressed feelings, and distance between where we each were living at the time: I wrote a letter, albeit somewhat sappy. The result of the letter was that she changed her phone number. I couldn't believe it, so much so that I called her mom and one of her friends to confirm that she was OK.
Six months or so later I happened to run into her, almost literally, in an aisle in Walmart in _my_ town while walking out of a cross aisle. Our eyes met, and given the glare I was receiving, I didn't know what to do. My feet kept walking, while at the same time feeling like my soul was being ripped out. How could she hate me so much? I didn't want to exacerbate the situation further, so I left the store.
Questions:
What would the rest of you have done? What do you do when you run into ex's where the parting wasn't amicable; do you say "Hi", flee, ignore them, stare at them until they speak or move?