Ever wondered what Kirk's pain was and why he refused to share it with Sybock?

Me neither.

But I suddenly developed a theory: In TVH, Kirk travels to the late 1980s, a time shortly before the Eugenics War and Khan's rise to power. If Kirk had intervened with Khan's power grab, he never would have encountered him in the 23rd century, never would have had the chaos with the Genesis device and the creation of the Genesis planet in the Mutara sector. His son would not have become entangled with hostile Klingons and would not have been killed. You see, the premature death of the tyrant Khan would have ultimately saved his son! But Kirk realizes that his pain is what makes him what he is. So it's okay that he forgot to kill Khan while picking up some whales.