To say the CorpseMen are stigmatized is like saying the Elemental Plane of Fire is a bit warm. They're not the sort of doctors you can just keep away by eating apples. Where they go, death and plague either follow or precede them. Granted that they are healers, and skilled ones at that, but their practice is queer among even the alchemical. Any herbologist could mix together a simple potion, but the CorpseMen are not just any herbologist. They can often be seen in the aftermath of deadly, brutal battles, picking apart the corpses of the fallen, taking samples and cutting pieces off. No one knows why they do this or what they are harvesting, but many suspect that it has much to do with their strangely specific toxins and reagents. They can heal or harm in equal measure, and there seems to be a needle specifically concocted to suit anyone and anything.
A rampaging troll on the countryside? There's a shot for that. Plague stricken farms? There's a shot for that. Necromancer got you down? There's a shot for that. Any creature, any person; if it bleeds, a CorpseMan can take it apart from the inside out, or stitch it up all the same. In some cases, the bleeding part isn't even necessary. That, however, doesn't stop the suspicious among the layman to call whatever it is they do dark blood magic. Most adventurers wouldn't touch a CorpseMan with the ten foot pole in their pack.
With their distinct masks, they're easy to spot, and their bird-like appearance inspires people to refer to them as carrion predators - "vultures" are a favourite among the caravans. No one has seen one without their mask. Perhaps they hide their faces in shame, or perhaps their identities are hidden for more nefarious purposes. Whatever the case, no one man has seen a CorpseMan's face and lived to tell about it.