Zargon's a hungry beast who only doesn't eat his followers because he likes being worshiped, transforms his followers (who are a cult of drug-addicted hedonistic madman who dwindle in numbers as time goes on thanks to their self-destructive habits) into mindless oozes the moment he doesn't need them anymore, and has an alliance with the (Chaotic Evil) Demon Lord Juiblex in the Elder Evils book. You'd barely need to change anything to make Zargon a Demon Lord.
So Chaotic Evil seems like the Alignment that fits him best.
I think Elder Evils only listed him as Lawful Evil because it retconned his origin so he was an Ancient Baatorian from the Nine Hells and so had to be Lawful Evil.
And Pandorym already exists as a Lawful Evil Elder Evil.
Or Atropus, but I think that's another case where the Lawful Evil Alignment doesn't make sense since Atropus is an undead world who blindly seeks to end all life everywhere, so Neutral Evil would make more sense for him.