Any semi-intelligent fool would be a tad wary after she's spoken again. The Master didn't get where he is today without being careful, as well as curious. If this is a warning, chances are she won't kill him - if she has that power. But there's definitely the Doctor's scent, and his presence. Perhaps she does.

He needs to know. One thing he already knows, the Doctor won't sanction murder. That's another lesson well-learned from 'the Valiant' days. “Very well. Though it wouldn't do to make an enemy of me. Maybe you should bear that in mind.”