A once in a life buck, Hummmmm. This isnt an easy one. I would probably shot my self and call it all good. My tendency would be to talk my self into it. Trying to justify it by telling my self I will never see one like this agian. I would eventually say no. Then I would make note as to where the buck was, do some scouting, and show up there agian, and hope to see him.
As much as I hate to I would let him walk. The main reason is there has been a line drawn, if I cross it, where will it stop.