She wasn't a kid- she was 21 years old, an adult by any standard. She was in Los Angeles, where she had many options. She could have left the baby at a safe haven, anything. Instead she murdered her newborn baby- how could she do it while holding her own child in her arms, a living, breathing being I will never understand.
The first birth is usually the most agonizing, and to be going through it alone must have been completely terrifying. There is often a good deal of blood lost, and if she had no medical attention, she might not have been thinking clearly.
I can't help but think that most of the "burn the witch" crowd posting here are men - who don't have the faintest idea of the physical toll of pregnancy and childbirth on a body. Before modern medicine, childbirth was one of the top killers of women.
This is a tragedy. No woman under the best of circumstances expects to go through childbirth alone. Something else was going on/not going on for woman/girl to have done this.