The ethics of the medical profession, as for any profession, are a set of rules imposed upon the profession's practitioners that are intended to preserve and perpetuate the delivery of the highest standard of service to the community. While there are public and private processes for the enforcement of ethical standards the individual's adherence to the ethical oath will ultimately turn on his commitment to the values and ideals of the profession.
And there we have the disconnect between Marc and the practice of medicine. Marc's attitude toward humanity in general, and his patients in particular, was so jaded, so warped, so disrespectful, that he was incapable of being a healer. And when you layer on to Marc's standard disregard for his patients his special animus for Ralph, the stage is set for murder.
There was no "negotiation" between Marc and his ethical oath in his approach toward Ralph. For Marc, his medical calling and the trust that Raph placed in Marc for his well being merely made it easier for Marc to carry out the dirty deed. While the act was perpetrated by a doctor in an exam room, the mindset of the killer was no different than it would have been for a triggerman in a back alley.
As for whether physicians should treat friends I believe that as a general proposition, probably not (particularly for the primary care physician). Professional distance is important in the doctor/ patient relationship. A prior personal relationship will inevitably affect the way information and counsel is given and received and the expectations that each participant has of the other.