What makes certain murders a "capitol" murder?

My ex husband was convicted of capital murder. The court said the crime was especially heinous and brutal. My ex husband murdered our 6 month old son. I understand the defining factor of a "capitol" murder to be a murder that is especially heinous and brutal, or a murder of someone who is 12 years old or younger. In my case, it is both. The ex husband was sentenced to death. This was in the state of Alabama. I believe this was a just sentence.