There is an even lesser sin that had God's people, under God's orders, kill pregnant women and children. In other words abortion is not remotely the worst sin. Idolatry is far worse. So when I see people who are Christless, Godless Idolators, the killing of their own babies is a lesser side effect that God may do anyway. The real issue is man's salvation. There are a lot of people on the pro-life front that forget this. I am careful not to cross that line. Sometimes prolifers mistake that as being hardened to the killing of babies but I try to keep all in a biblical prespective.
Abortion controverts Gods command to go forth and multiply. It, abortion, diminishes our understanding, accepting, and bowing to Gods sovereignty, by elevating our desires above Gods will. To not recognize this is to deny the most basic concept of idolatry: elevating that which is not God to godlike status. Abortion elevates the desires of self to a position superior to the will of God.