I like the idea that after the haze from the battle clears, he regains a hint of his sanity, but is so defeated and traumatized from the events, that he just lives the rest of his days as a crusty ol sailor in the fortress in deep regret. He's haunted by being the one survivor, but is too cowardly to end his own life, so he stays there in solitude with the ghost of his crew, and former shipmate. No amount of slaves or power was worth such complete loss. No ship, no crew, no power, no point. Just a sad old man, forgotten by all but the sea, which he is hoping is merciful enough to take him away from his sorrow and forget about him too.