
...I'd rather not have an entire argument about this. She could have refrained from attacking as we were talking to the ghost, she didn't, he stabbed her in return. He could have refrained from mortally wounding her, yes that's bad and yes that means he's an ass.
However according to that prophecy thing Rumbottom quoted he needs to sink on an unstolen ship, so if you destroy the ship to keep it out of his hands you're not fulfilling the prophecy because you're going out of your way to ensure he can't have it and he's just going to keep haunting. He might just keep attacking ships until he encounters one where people are willing to surrender their ship in exchange for their lives.