Sinspawn are a symbol of Pathfinder’s glorious wild and wooly first year, when Varisia offered a new and dark playground for D&D 3.5 to explore. Like so many things in those early issues, sin was at once both a radically new and perfectly old way of thinking about the schools of magic and about evil in general…and sinspawn were delightfully disturbing and bestial new monsters that were equal parts pulp fantasy and anime. (Just look at those jaws!)
That said, taking them out of that context takes some work. Unless you were raised on a particularly grim branch of Christianity or have a deep fascination with Dante and Milton, the Seven Sins probably aren’t top of mind for you on a daily basis. And sin as an organizing concept takes a back seat to alignment in most RPG worlds.
So: What is sin in your campaign? Who is crafting these spawn? What souls or terrible sacrifices do they require? What makes them so adept with martial weaponry, and is this ability inborn or trained? And to what ends might this created and sterile—but intelligent and driven—race by put to?
A lamia has found an artifact that will allow her to create sinspawn, and with the aid of her first creation, a lustspawn sorcerer, she is ready to begin building her army. She has refined the rite as well, using orcs to create vile gluttonspawn skilled with serrated falchions and a penchant for raiding on feast days.
In the sternly cosmopolitan city of Carthex, diabolism is considered as valid as the veneration of saints, and often more useful. Dark clerics use sinspawn both as minions and as reminders of the danger of indulging in vice without infernal protection. For instance, they send lustspawn on raids of unregistered brothels, and no sale of platinum can take place without giving Mammon his tithe…or his greedspawn will claim it and tenfold more in the dark of night.
Nobles and heroes in the Jade Empire tainted by dishonor risk possession by an oni or returning from the grave undead. Common folk risk a meaner but equally horrific fate: transformation into a bestial sinspawn, the type determined by the particular appetite that led each offender off his or her Virtuous Path.
—Pathfinder #1 90–91 & Pathfinder Bestiary 2 246