The idea, with an ideal justice system, is that for a truly heinous crime, the culprit should be punished in the most severe manner available. To me, being put to death would be quite a punishment. So, you could argue that for certain crimes, the perpetrator should be put to death.
Abortion is the killing of an unborn child (in some cases at least, gotta define 'child'). The child is innocent. The innocent should not be punished, so the innocent should not be killed, so the baby should not be killed, so the mother shouldn't be allowed to have an abortion.
So here we go: death penalty is justifiable, abortion is not.
Note that I'm just playing devil's advocate here, these are not necessarily my opinions.