I like your idea, and it certainly makes more sense to me... but I think this would make things way too complicated. Mostly because the ability 'devour' is already defined as being dependent on the amount of HP the creature has, so we'd have to change the ability completely, and define it all within the card text.
Unless you can propose a viable solution, I think Scarab is better off staying as it is...
Edit: After some thought, I think I'm agreeing with the other two responses, so far. Although I still like your reasoning, I don't think there's anything innately wrong with Scarab, and I think it's just fine the way it is now.