Just going off of the emotion she felt from him, Mara knew this was a dangerous situation. Obviously something had gotten to him, and oh she knew what it felt like to just want to turn around and unleash the force of that emotion on someone. It was a difficult thing to resist when one was trying to move away from the Dark Side. Even so, she wasn't going to let him take a step back, no matter what was making him react like this.
When Galen said he was going to put an end to Moriarty's men, Mara was slightly confused, though she didn't let it show on her face or in her body language, which remained calm and neutral. "I understand that Moriarty's men need to be taken care of, but that does not explain the anger I feel from you." She had a severe dislike of Moriarty and his men, especially for having strapped Jaina to a bomb, but the anger she felt over it was nothing to the extreme that Galen felt it. So there was obviously something else going on here.
"I will not because I won't let you go through with this, not in the current state of anger you are in."