Can God's children behave foolishly? Can we be unfaithful, manipulative, insensitive, selfish and just plain mean? I'm afraid so.
Abraham and Sarah tried to accomplish God's will their own way - and they made a mess. Their relationship with God, with each other, with their household and between the nations (that were to spring forth from Abraham's loins) was dismally affected by Sarah's foolishness and by Abraham's weakness.
Abraham was only human. He was in his 80s and still had no children. His sweet and humble wife strangely offered to let him have sex with her younger Egyptian servant. He foolishly went along with it. But, this was not the way God had chosen to bring THE SEED forth from Abraham. The seed (which is Christ) was to come forth as a result of promise, faith and power... not promiscuity, fornication and a paramour.
When we leave God out of our life-equations we always come up with the wrong answers. God had rejected Abraham's idea of settling for one of his male servants (chapter 15), and God (not surprisingly) would also reject Sarah's idea of settling for one of her female servants. The great patriarch and matriarch of faith should have just waited upon the Lord. Instead, they rushed ahead... hurting themselves and others.
Beautiful Sarah wasn't quite so very attractive as she exercised cruelty upon Hagar and upon Abraham. Can't you hear the bitter bite in her words to her husband, "My wrong be upon thee!"? And, notice how insensitive Abraham and Sarah became toward Hagar in all of this. I'm not saying Hagar was innocent and pure, but one would expect more from these two elderly saints, right? Yet, we still see this problem today too, don't we? People who ought to know better can do a whole lot of damage when they lean on their own understanding instead of simply and humbly acknowledging GodAnyone; everyone is susceptible to making this kind of a mistake! We are also all so very human! And that's NOT a complement. The old song says, "The arm of flesh will fail you; you dare not trust your own!" Depend on God, not on self! If we depend on ourselves, we will end up hurting someone for sure.