Well, that's because in the minds of many, killing a wholly innocent partly or fully formed child is fundamentally different from killing a serial child rapist or a cop killer or a mass murderer. You may not be able to see the difference, but most people do. (I am against both, but I can clearly see the difference between the two!) What's harder to explain are those (liberals) who are up in arms about the death penalty (saving serial child rapists), while promoting and supporting things like partial birth abortion and even letting babies die after they're born, if they were born during an abortion procedure. For them, things like second hand smoke are more important.