Though Sebastian's first meeting with the will of the legendary Sea King of the Black Veil was very little and was only through projected writing, he could not help but have his own opinion of the Sea King already.
The little that was revealed by the will already told him much about its thinking faculty and the type of personality that it was.
From Tonado, he already knew about wills that were left in inheritances by powerful predators. In essence, they were the consolidation of the said predator's obsesses during its living days.
From the little that was revealed to him, Sebastian concluded that the Doppelganger and Invisibility Sea King was a crazy, arrogant, carefree, extremely lunatic, and tyrannically dangerous old predator.
The only missing puzzle in his conjecture that prevented him from peering into the soul of this Sea King through its will's writing was the fact that he didn't know the main species of the Sea King.