The relationship between Sara Imeinu and Hagar, was strained from the start. It was actually her idea that Avraham take her as a wife when it was clear that Sara was barren.
It appeared that Hagar was a good choice. She was the daughter of Pharoah, a princess, who preferred to be Sara’s maidservant rather than live the life of luxury in the palace with her father.
Nevertheless, there was friction from the start. Hagar flaunted the fact that she conceived immediately. The Midrash says that Sara gave her an Ayin Hara, and she lost that baby.
When Yitzchak was born, there was an age difference of thirteen years between him and his older brother, Yishmael. As the boys grew up, Sara did not want her precious son to be influenced by Hagar’s son. Even as a boy, he was demonstrating signs of wild behavior, that would not have a good effect on Yitzchak, the heir to Judaism.
Avraham was disturbed by Sara’s demand to throw both Hagar and Yishmael out of the house. It was only when Hashem intervened, that he realized that his wife was a prophetess and she knew what was best for Yitzchak.
Many of the commentators such as the Baal Haturim, felt that Sara’s biggest mistake was her asking for judgement. She told Avraham that Hashem should judge between her and Hagar. It will be proven that she was right in her treatment of the Egyptian princess.
Asking for judgement, or “Din,” is almost like a death wish. We could not survive if Hashem ruled the world with Din. Fortunately, He rules with “Rachamim,” or mercy. The Baal Haturim went so far as to say that asking for Din, is like committing suicide.
Nevertheless, we do not find any blemish on Sara’s record for her harsh treatment of Hagar. Apparently, her intuition and gift of prophecy, guided her correctly. Avraham was wise in that he listened to his wife, despite his own reservations.