While I do agree that it wasn't the typical mocking sarcastic line uttered by a character who managed to entrap another, I think it was a bit rarer sad sarcastic line of farewell. She was saying goodbye to him, but since she had to force herself to betray him (had to convince herself to do it; he's an evil adventurer just like the others), that line was just one more part of process of getting over the regret she felt. Maybe she even deep down hoped he might survive and punish her later, who knows how deep her regret was, but she didn't really believe it.