I'm of the opinion that killing murderers and rapists isn't punishment. You learn from being punished. You can't learn if you're dead.
This assumes that some rapists can/are willing to learn and/or sane.
It is, however, revenge (especially with comments such as 'because they deserve it'), and I am of the STRONG opinion that vengenace has as little business being a part of the judicial system as religion does.
Vengeance is limited in effectiveness and easily manipulated. If a rapist dies then he or she can not harm anyone else, this is not enough. It is the caveat of wishing for all the rapists in the world to die. It stops the immediate problem but does not prevent rapists form existing in the future.
Preventative measures, in my opinion the creation of a biopsychosocial framework, are a better fit for justice. Being that convicted rapists can be forced into a study (as compared to execution) than an ample population is available for research.
To be honest I would prefer to see both a study and death occur, after all take no chances.