Demonic aesthetics

My problem with the demons and devils is that they lack unifying motifs. Each side is a grab bag of random designs and their hierarchies and behavior are more similar than not. I have a few different ideas for explaining why demons look and act the way they do, and making a clearer distinction between demons and devils.

For the sake of terminology, I am not going to arbitrarily divide fiends, demons, devils, etc unless I am specifically pointing out differences between them.

Brood: a concept introduced in Encyclopaedia Arcane: Demonology and The Slayer's Guide to Demons, all demons of a given brood share a similar physical aesthetic and capabilities. For example, the Vang are hive-minded insectoids while the Jartaska are made of shadows and light.

Caste: a concept introduced in Infernum: Book of the Damned, caste is a distinction between demons of the same brood who have been created or modified to perform specific tasks. For example, a stalker hunts down prey while a slaver tortures and molds damned souls.

Sin: Building demons around the seven (or more) deadly sins is an easy way to design behavior and aesthetics. There are several directions I could take this.

• Does the demon personify the sin, its opposing virtue, or its opposing virtue taken to a harmful extreme?
• How does the demon's appearance relate to its motives and behavior? Is it ironic and deceptive or obvious and expected?
• How does the demon interact with its sin? Does it indulge the sin, cultivate the sin in others, or punish sinners?

Some combinations display synergy, some not so much. For example, one group of demons could personify and indulge their sin, another could personify virtue while cultivating their sin in mortals, and a third could personify an exaggerated virtue while punishing sinners in hell. (Personifying virtue has the benefit of a better work ethic.)

I decided to use this as the basis for a division between three ethnic groups in my version of the lower planes. The lower planes are inhabited largely by demons which simply personify and indulge their sin, while the devils are distinguished by personifying virtue and torturing the damned. The third group personifies virtue and cultivates sin within mortals, and are often conflated with devils. This isn't a hard and fast rule, but it helps to distinguish them more clearly.