I think the only natural predator in the SE US that would kill an 8' indigo would be either an alligator or a Florida Panther. (which there are like 12 of). I don't think even a bobcat would mess with a large adult. Easier meals are all around.
Of the snakes with transmitters in FL and SE GA, there has been only one "suspected" killing by a predator. It was a 5.5' snake that found itself entangled in a chain link fence and was killed...either by a large bird of prey, a bobcat or a pet.
I'm sure in the US there are many more adult indigos killed by dogs and humans than by natural predators.
I can't speak for Drymarchon in other parts of the range...but for the most part, I believe that large adults are at the top of the food chain.
Young Dry's of course fall prey to many predators.