[Law wants to believe Olivia would draw the line somewhere, but honestly, he has yet to see her do it. After a tense pause, he does answer.]

Because of a debt I needed to repay.
[He doesn't really want to go into it here, but he's worried that she won't believe him if he doesn't give a less vague answer. When he continues, it's clear in his measured tone that he's picking his words carefully.]

The man who saved my life was killed by a powerful pirate. I wouldn't- [Stops, starts again.] I did what I had to do in order to defeat him.