Star Trek: 10 Things You Didn't Know About Dr Leonard 'Bones' McCoy

In The Final Frontier, Spock's brother Sybok used his unique ability to tap into people's deepest pain to force McCoy to relive the death of his father, David McCoy.

David was diagnosed with an incurable disease and, after living for too long in constant pain, pleaded for his son, Leonard to take him off of life support. Leonard didn't want to watch his father die slowly and painfully, so he disconnected the life support system to 'preserve his dignity'.

Leonard had regrets about his decision, but the worst pain came when, shortly after David's death, a cure was discovered for his disease. If Leonard had not deactivated the life support, then his father may have lived much longer. He continued to regret his decision for years.

Sybok's intervention seemed to help McCoy to move past his pain. If nothing else, it helped him realise that his father's death was not entirely his fault.