In my experience, only very rarely have they been "malignant narcissists". More often than not, they actually had a valid grievance; I do have a marked tendency to assholery.
First, I listen. I listen carefully to that they're saying, and resist the urge to shut down or become defensive. I try not to interrupt, deny, gaslight, or divert them onto a more comfortable tangent. I listen, without simultaneously composing my reply in my head. I fact-check; asking reflective questions like "you mean _______?", or "what I hear you saying is ______?". Everyone wants to know that they're heard, that they're feelings and concerns are taken seriously, and quite frankly, I suck at this and need the feedback.
If their complaint is valid, I apologize without invalidating it by BS self-serving rationalizations or lame attempts to justify my behavior. I try to do so plainly, succinctly, and sincerely, incorporating an acknowledgement of their feelings; again, people need to feel like they're heard, and feelings of insult put relationships upon thin ice; at such times absolute candor and clarity is needed.
I try to learn from their feedback and improve the way I relate to others, but clearly I'm a particularly dull pupil.