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Kozmotis Pitchiner won, he got his vengeance, he killed every last Dream Pirate on that ship, beheaded every single person who had a hand in the attack on his home, the death of his family.
It wasn't enough.
He fought a war, left a trail of destruction so wide through the ranks of the enemy that the battle that had lasted for centuries was over in a few short years. Every Dream Pirate and Fearling not brought to ruin under his hand was corralled in a prison so deep no crack of light or drift of fresh air could enter.
It wasn't enough.
Kozmotis Pitchiner wanted vengeance, and he got it. He wanted to see them dead, and killed them, wanted to see to it that no one would ever be hurt by them again, and succeeded. He wanted to place himself at the door of their prison and bear witness to their eternal torment, sure in the fact that they could never be free from the fate he had personally sealed for them. He was granted such.
It was never going to be enough.
Kozmotis Pitchiner is the hero who won, the man who saw his family slain and fueled that rage and grief into his quest to destroy all evil, and he did it. No one stopped him, no young plucky protagonist stood in his path and told him sliding his sword through the necks of every last Pirate wouldn't bring his family back, no good natured lifelong friend talked him down from that ledge, no one on the side of the Just and Noble and Right ever tried to stay his hand.
Kozmotis Pitchier eradicated the Dream Pirates in their entirety, and it simply wasn't enough.
He didn't lose his daughter and suddenly decide to burn the whole universe to the ground. He got his vengeance, and it wasn't enough, he protected everyone else in the living realm from suffering the same fate, and it wasn't enough, and finally when the war was ended and he should be at peace he placed himself in the position where he could watch the endless suffering of his enemies, and even that wasn't enough.
Pitch Black is a being crafted from a man who at every turn won, but all that slaughter never did the one thing he needed most, it didn't bring him peace.
His daughter was alive, adrift in space, and he never looked for her because he never stopped to question the information he had been given. Never stopped to investigate their deaths, never noticed the missing schooner, the missing life-sized doll, never tried to recover their bodies? Just believed the words of the captured Dream Pirates without question and lost himself to the quest for his vengeance.
In every possible way Kozmotis Pitchiner has been losing the war inside himself, and instead of stopping to wonder if maybe he has been wrong all along he continues on, pushing further and further into the darkness.
Pitch Black murdered his way across the cosmos precisely because Kozmotis Pitchiner could never be satisfied with vengeance alone. There was no point to a universe where his daughter no longer lived.
Learning she was alive the whole time?
Well.
That's enough to drive anyone mad.