The Master's return in series three / twenty-nine's Utopia had a profound impact on the Tenth Doctor. He saw his beloved humanity enslaved. He found himself imprisoned and tortured. And he faced the conflict of interest that was discovering another member of his race alive, even though that member was his enemy. The Master's plan was particularly cruel; he obtained the last remnants of humanity from the distant future and used them to conquer the humanity of early 21st Century Earth. But he also took a leaf out of the Doctor's book. This time, he had his own companion... his wife. Ultimately that was the Master's big mistake. Lucy Saxon began as a power hungry woman with no scruples. She knew who her husband really was but she supported him, first as Prime Minister Saxon's wife and later as his companion over a conquered humanity. But his actions had an impact; over time she saw his hatred and aggression against her own people and became filled with regret. The Master begun to abuse her; whether it was physical (it was certainly implied) or simple neglect, she found herself increasingly isolated. So when the Doctor and Martha finally defeated his plans and reversed time, she struck, shooting her husband in the chest and killing him... at least for a time. The moral of this story? Don't anger your wife... she's far more dangerous than your mortal enemy!