The last the crew saw of Bill Turner, he was sinking to the "crushing, black oblivion of Davy Jones' Locker," and was left to suffer a life at the bottom of the ocean. Although he was eternally drowning, Bootstrap Bill was unable to die because of the Aztec curse.

Why did the crew kill Bootstrap Bill?

Barbossa kills Bill to maintain order. Barbossa knows that Bill has cursed them forever. But he doesn't know about the immortality (yet). He drowns Bill because he wants him to die.

Why did they need bootstraps blood to lift the curse?

Infuriated by Bootstrap's betrayal, Barbossa and his crew had looted a chest of Aztec gold. ... It was only after they had thrown Bootstrap overboard that they learned how to break the curse: they needed to return all of the gold pieces, and each pirate who took the gold had to make a blood offering as a form of repayment.