Headcannon: Beel's "Warden of Decay" moniker isn't just for show or her self-aggrandizing. It's literally one of her cosmic functions and jobs. Her very existence causes things around her to break down and decompose. To wither and rot so they can be recycled back into the cycle of creation, death, and rebirth. Simply being around her, living things fall ill and waste away. Metals rust and corrode. Matter falls apart at the molecular level. It's why she ended up being at odds with Lucifer despite them both being "demons." Though fallen, Lucy is still an angel at heart. One of the Almighty's top and favored creations. An avatar of order and structure as reflected in her strictly hierarchical and even corporate inspired Hell. For all the evil she does, Lucy does it in a painfully rigid and organized manner. Hell itself being weaponized stagnation where nothing changes or grows. So it was inevitable she'd come to blows with Beel. They sit on diametrically opposite ends of the philosophical spectrum. It's why she's so dangerous and could only be contained, not destroyed. You can't "kill" the fundamental force of entropy itself. And since the Abyss is an absolute absence of light, matter, time, energy, etc her powers have no effect on it. It's the only place in the universe capable of holding her and she would indeed more catastrophic than Loremaster if she escaped (Loremaster only really dialing up that whole "order" thing higher than Lucy ever could.) And it's why Heaven itself shows props to Justice for taking her down by referring to her as "The Flybreaker." Beel's very existence was a threat to both Hell AND Heaven. Maybe not intentionally. Beel herself seems too self-absorbed and lazy to actively stage a coup, more interested in her personal pleasures and pass times. But she also seems to have quite the appetite, metaphorically speaking, and may have been indulging herself to the point it made everybody above and below realize she needed to be stopped. So in all those regards Beel really is "The worst kind of demon" because while Lucy and Co may unleash Hell on Earth, at least they'd leave an Earth to have Hell on. Beel would eventually utterly destroy it.