If you cast purge on a target, it looks for *enemy* controlled cards to destroy, that is, cards cast on the target by the opposing side. Festering Guts was cast on the target by the same side, so those cards are not considered a target for Purge. I see where the confusion comes from, the use of "enemy controlled" leads you to think that Festering Guts is an "enemy" card.
This will be fixed in the new build - so that "enemy controlled" means "enemy of the caster" not "enemy of the target".
