The tragedy is not to die, but to be wasted.
Leonard McCoy is a doctor, and a damn good one at that. Starfleet knows it, he knows it, and he knows Starfleet knows it. That being said, it’s no surprise when he’s assigned to the ISS Enterprise. It’s not a surprise when he sneaks Jim Kirk aboard, either.
It’s even less of a surprise is when Doctor Puri’s body is found in a medical supply cabinet, no marks on the body. McCoy inherits the title of CMO and he holds it. Anyone who comes near him suffers the same fate as Puri.
(Unless Jim Kirk finds him first, but that’s a different story.)
McCoy knows his strengths and his weaknesses as well as he knows anyone’s; he’s not afraid to stand up to Spock. He also knows how far he can push the Vulcan before he snaps; Spock may have the best poker-face, but McCoy?
He’s got heart, and in this game, that’s the only card you need. He kills quickly & cleanly; too much so to cause any horrible pain - not that he can’t hurt, because he knows exactly how to make someone break in less than a minute.
But he’s also a doctor; his job is to save lives, because those lives are still useful to the Terran Empire. The dead are no more than wasted opportunities. They’re not keeping score, not officially, but he’s saved the most out of all the Starfleet doctors in the fleet. He saves their lives; wins their loyalty.
Cold and cruel medical precision coupled with intelligence and southern charm make him the most dangerous man in the fleet. Leonard McCoy may save lives; but cross him, and you’ll be gone quicker than you can scream.
And yet - he’s empty. There’s nothing in his eyes; he’s just a shell. His soul has long since departed; his conscience exists no longer - if it ever did at all. All that is left is his skin & his bones.