My reasoning is that there is a very basic sense in me that reciprocal punishments are fair, and that getting vengeance for a family that has lost a loved one is not a bad thing. While some might claim that vengeance doesn't make the aggrieved family feel any better, I'd suggest that it makes a lot of people feel at least a little bit better (though it doesn't completely compensate for the loss obviously.
To me, denying that vengeance for the family of the murder victim is a good thing is simply to deny a basic (if ugly) fact of human psychology.
As for which crimes warrant the death penalty, it would have to be deliberate killing, without mitigating factors (or a lesser form of murder if there were aggravating factors). "Heat of passion" crimes have some mitigation, because by their nature the killer was not thinking clearly at the time of the act. There I would balance the desire for vengeance of the family against the need of individuals in society to know that they will only be condemned to death for the actions over which they had control/
A rapist killing her attacker would also have a mitigating factor. Again, people tend to find reciprocal punishments "fair." If a rapist killed her attacker, that might be going beyond reciprocity, but not far beyond it. So the punishment inflicted is only "a little unfair" (Again, though, this is assuming no error on the part of the killer.)