I like Sarah. She followed her man all over the place. Many wives would have had concerns at starting a nomadic existence at 65, with the promise of children thrown in as well. I have portrayed her as a beautiful woman rather than aged. She must have been ravishing as twice Abraham asked her act as his sister when they went into foreign territory, and both times the rulers took her to be their wife. And this when she was in her 70's and 80's!
I can understand her behavior regarding Hagar, she longs for a child and wanting to carry on Abraham's line she gives Hagar to him. It is too much for her when Hagar treats her with scorn for not being able to conceive. Being unable to conceive is a terrible thing for any woman to come to terms with, (I only know too well as that is my story). When she finally bears Issac, she is like any protective Mother, perhaps more so, as she had to wait so long to be a Mother. The mocking behavior of Hagar's son is the final straw. She has to get rid of them.
This story is really about God's Promise, and waiting for it. Sometimes, as here, we have to wait years, and the trick is to not go and try and contrive an outcome before its time. This is why, even when it seems improbable , such as Sarah conceiving at 90 years of age, we have to wait on God's Promise. If our Father says something is going to happen, it surely will.