If that's true, why was Scott Peterson convicted of killing two people, Laci Peterson and her unborn child? Wouldn't that make the unborn child a person since he was convicted of killing two people?
Scott Peterson was not convicted for killing "two people", but rather of killing his wife and her unborn child.
While the second conviction is constitutionally dubious, the prevailing reason it is able to stand (for now) is that in the USA, only Mrs Peterson has the legal option to terminate the unborn fetus. If someone else does it against her wishes, then a criminal act has occurred.