That's a good question. I guess this makes a difference if a hero has retaliation damage; then the villain with the highest hp can change over the course of the damage-dealing. Both readings are possible in English.
I think it'll be the same villain, otherwise it will become very troublesome to check if cards like Combat Stance is in play. So if Get happens to be the highest and a card says the highest deals all heroes damage, then Geb will be the one to carry out every hit.
You know, I know that. . . and yet somehow that never translated into my brain for the Ennead. I've done that numerous times, and never thought anything of it.
I don't think it gets to change since it uses the singular "Villain target". It finds the valid target (your choice if there is a tie or whatever) and then each hero target gets the damage coming at them from that source. Unless something were to end up redirecting the one shot somehow, my understanding is that targets do not change once they have been chosen and as written on the card, you choose the villain target before you start dealing damage.