That is precisely what defines these people as "conservatives." The U.S. Constitution includes a clearly-defined process for passing amendments. What infuriates conservatives is that liberals have never even bothered to amend the Constitution -- they simply rely on justices to render decisions that have no basis in Constitutional law.
I'm no fan of the death penalty, either. Not for any of the reasons that you describe, but because a nation that can produce an O.J. jury, a Clinton presidency, or a Jerry Springer show doesn't have the moral authority to put people in jail, let alone execute them.
What is unusual about that? If you think the Constitution has a flaw, the proper way to resolve it is through amendment, not by re-interpretation based opinion polls or popular sentiment. That is not law --- it is mob rule in robes.
I personally have a problem with the death penalty, not on constitutional grounds but on religious/moral/ethical grounds. But I refuse to insult anyones IQ by making a case that it is unconstitutional on its face. It clearly is not.
This ruling by the SC is bizarre at best. The perp admits forming prior intent and admits understanding that it is wrong. I don't give a damn if his IQ (what the hell is that anyway, and who decides?) is 70 or 170, he meets the criteria for pre-meditated murder in the act of committing a felony. If I get caught I can simply plead stupidity while someone doing the exact same thing who happened to finish high school still gets fried? Are we equal under the law or not?
In addition to what others have said concerning amendments being the right way to change things, many of these proposed amendments are designed to suplement, not change, the Constitution and make it nearly impossible for the Supreme Court to "interpret" it however they feel.