This is because, even after all the incredible miracles of preservation and protection God had performed on their behalf, they kept trying to decide if they could really trust God; if He would really keep His word and bring them into the land of milk and honey as He had promised Abraham.
Awww, poor birth mommy might feel sad she gave her healthy, unwanted kid to a couple who have been praying and hoping for a baby. Oh no, mommy shouldn’t go through any emotional pain, just because she became pregnant as a result of her own actions and carried that baby to term! Mommy would be free from all guilt and longing if the baby just went away. Yay! But how do we make the inconvenient baby go away? We kill it.