Didn't Mizora in no vague terms promise him an end to their contract if he saves her? Before she likely wanted him forever, possibly as just another soul to gain power, or I am guessing because of the situation, something more like a personal blade or lover for all eternity.
But now survival took precedence and if the deal is binding, Wyll can get free from her just by saving her. But, he could go even farther, get revenge on her, and that might be considered a hateful action, even if she is a devil.
