Rogue Assassin, by Timothy Kong
The Master uses charm to defuse tense situations, and goes ~bananas~ to put people off-balance and make them underestimate her. It's hard to take a death threat seriously when it's part of a Secret Girl Plan, and also makes her actions harder to predict. Saxon may be chuckling and eating jelly babies, but don't think he's not also scanning the room for threats. In The Magician's Apprentice, a line was cut where Missy describes herself to Clara as "a perfectly sane recreational killer." Yaaaaas.