In the Star Trek series, Leonard "Bones" McCoy usually serves as comic relief and as a contrast to the uptight Spock. Things are no different in The Final Frontier—but here we get some serious character revelations to flesh out the guy's inner life for us.
Like everyone else in the film, McCoy is suffering from some secret emotional pain. McCoy's pain revolves around his father. As revealed during Sybok's telepathic therapy sesh, McCoy pulled the plug on his dad when he was deathly ill and begging his son to "release" him from his pain. And that's what McCoy did, though not without protest. That's obviously a controversial issue, but no matter where you stand, we can all agree that this memory weighs heavily on McCoy's conscience.
We hate to say it, but that's just the start. Take a look:
SYBOK: That wasn't the worst of it, was it?
SYBOK: Was it? Share it.
MCCOY: Not long after, they found a cure. A goddamn cure.
Whoever wrote this bit is brutal. McCoy must reckon not only with the fact that he contributed to his father's death, but that he might have recovered if McCoy had simply stood his ground. How are you supposed to get over something like that?
Apparently, the answer is "befriend a weird Vulcan cult leader." For as much as we can criticize Sybok, we have to give him credit where McCoy is concerned: he truly helps the guy release his "secret pain." This might explain why McCoy suddenly becomes so loyal to Sybok after their session is complete.
In the end, of course, McCoy returns to the good team. Still, if there's one positive takeaway for him at the end of the film, it's that he's finally begun the healing process after his father's death.