Good point about the common cultural background to the Death on horseback imagery. I think that a lot of these interpretations go along with the BGGB genre. He's a scary, bloodthirsty, half-mad horseman, but just as "the moonlight shows us for what we really are" in POTC (i.e., not the ordinary mortal humans they appear to be), so the lightning shows this horseman to be an incarnation* of Death, something far more dreadful than a merely human villain. It's very difficult for me to see the transformation as a comeuppance, a stroke of divine justice striking him down, a reminder that even the strongest man is mortal, etc. The skeletal form is still too vigorous and undiminished. If the latter readings were the sort of thing Marc intended, I would at least expect to see the rider's hand opened, losing his grip on his sword, an easy and obvious way to indicate loss of power. You see this all the time in comic book art, etc. It's a cliché. A guy gets shot/zapped/whatever, and at that very moment his hand weapon is shown floating for a microsecond in mid-air, still there between the now-opened fingers and thumb of the hand that was gripping the weapon a microsecond earlier. Such an easy way to show us that the horseman is going down.
*Hmm, on reflection, "incarnation" seems a poor word choice, as there's so little carne left here. Maybe "avatar" would be a more appropriate term.