1-The Doctor Shipman
On the surface, the Doctor Shipman is a popular doctor, good at his job and with an excellent reputation. But a few of his fellow doctors are worried about his abnormally high death rate compared to other doctors, and would be shocked if they knew the truth. For several years, he has been secretly killing his older patients with lethal injections, falsifying their medical records and often signing cremation forms to dispose for good of the evidence. As he only targets the old and the ill and never tries to steal anything from them, so far he has not been found out, and perhaps he never will.
Red Cross belongs to an order of doctors that operate in war zones all over the world. They treat the wounded of both sides equally, never carry weapons (although they often carry surgical tools with them, these are not seen by the rival sides as weapons but just the tools of their trade) wear uniforms to show who they are, and never spy. And as long as they keep to these strict rules, they may travel anywhere within the war zone. Skirmishes and even full scale battles have been known to stop for a while so that they can treat the injured. They also vaccinate the babies and children against disease.
The Medicine Man is a tribal doctor but that does not mean that many of his cures are not effective, and he is a popular and powerful member of the tribe with a limited amount of political influence. Since his tribe is a warlike one, allways trying to subjugate the smaller weaker tribes in the area, he is often called upon to bandage wounds, set fractures, and the like. Against the local diseases of the area, his cures often work, but they would be no good if explorers from other continents bring their own diseases into the area.
The Mengele works at a concentration camp where he perverts his undeniable medical skills to inflict horrible painful experiments on the helpless imnates. If encountered in a fantasy campaign, one of the things he might be doing is creating or at least trying to create new forms of controllable Undead things.
Quack Quack has in fact never been to medical school, although he does have some medical knowledge and is not totally useless. He travels from place to place to keep out of the reach of the authorities and those patients that his limited skills were unable to cure. He has a good bedside manner and often cons people into thinking he is much more skilled then he really is.
Outlaw Doc has ties to the criminal underworld, and treats all the gangs equally and skilfully for the same flat fee, no matter who they are or how high or low their *rank* in crime. If one gets wounded in a fight with law enforcement or in a way that would draw unwelcome attention from law enforcement if they went to hospital, they can go to him instead. And because he would never inform on anybody, and treats all the gangs, none of them would murder him. If he was murdered it would be a great loss to all the gangs of the city as then none could be treated by him.
He means well and is a skilled surgeon, but he works in an era where bacteria and other germs are not known about. He works with dirty hands and instruments, his bandages are not very clean and his aftercare is poor. Patients are unlikely to die on his operating table, but death from infection afterwards is quite likely.