Why did Dr. Livesey say that the tattoo near the captain’s shoulder seemed prophetic?

A fair wind," and "Billy Bones his fancy," were very neatly and clearly executed on the forearm; and up near the shoulder there was a sketch of a gallows and a man hanging from it--done, as I thought, with great spirit.

"Prophetic," said the doctor, touching this picture with his finger. "And now, Master Billy Bones, if that be your name, we'll have a look at the colour of your blood. Jim," he said, "are you afraid of blood?"

From the text, we can infer that Dr. Livesey recognized the Captain as a criminal, meaning the prophecy (the gallows) might be his end.