ksmith | Last of the Time Lords question

So I watched Part 2 of LotTL last night, and the same thing bugged me this time as it did the first time I watched it--what drives Lucy Saxon to shoot the Master? The only possible clue I see is the moment when she chimes in as everyone chants. Is that supposedly the point where all his brutality finally makes her realize he needs to be stopped? After all he's done, that's the point where she decides to stop him? Unless I missed a scene, that came out of the blue. An earlier look of disgust or fear or realization followed by loathing would have provided enough set-up to make me buy it, but either it wasn't there or I missed it. Did I miss it, or is it jut one of those story hitches to accept and move on from?