I guess I can't see past the simple point that a child 5 seconds from birth is eligible to be killed, whereas 5 seconds afterward is bestowed with some magical right to life.
A legal line had to be drawn somewhere, and the current line is drawn at birth. We bestow personhood on the human after that human leaves the mother's body while still alive.
We all agree (I assume we do, since it would be odd not to agree) that the human fetus is alive. The issue is whether that human is given basic human rights before or after birth.
Many states have moved the line backwards to a few weeks before birth, since an abortion after that point involves removing cut up pieces of what obviously looks like a tiny human baby. The human is still not considered a person or a being yet, that is still reserved until birth, but the human is given the right to life at that point.
It is an ongoing debate, and one which will continue as we learn more and more about our own species.