Try this on for size.
An HO is when a pilot -- having significant time, speed, energy, position and/or a positive or neutral tactical situation to try and work for any other gun solution -- clearly decides, with intent and malice aforethought, to go for the 0-0 aspect shot.
Lacking (one or more) these positive criteria (especially time or tactical situation), a pilot taking such a shot would be considered taking a clearly respectable angle deflection shot (if slightly off the 0-0 aspect by some degree), or an advantageous and 'unexpected' firing opportunity, even if he was at 0-0 aspect at the moment of the shot.
In other words, if the guy has a calm, not under fire, some distance away moment or two where they could plan and execute any other manuever for a guns solution, but instead just bore in with lethal intent, then that is what I call a HO.
Anything else, is a valid, snap choice, right or wrong. No fame, no shame.
And in EITHER situation, if you get killed, it's your fault, not the other guys good or bad tactics, lame ACM, or lack thereof. If it's my definition of an HO, you had time to get out of the way. If it was anything else, your ACM wasn't good enough at that moment to prevent your dying. All your own fault. Live (or die) with it.