The one reason that is complete BS enough for me to add a writeup is:
Capital Punishment is cruel and unusual punishment. If killing someone isn't cruel, then I don't know what is.
Capital punishment is very clearly NOT considered cruel and unusual punishment. The constitution, the same one which forbids cruel and unusual punishment, makes specific mention of capital punishment.

5th Amendment
No person shall be held to answer for a capital, or otherwise infamous, crime, unless on a presentment or indictment of a grand jury
Webster 1913 defines capital as: "Having reference to, or involving, the forfeiture of the head or life; punishable with death; capital punishment"

So just three amendments before that denying cruel and unusual punishment, the constitution specifically allows capital punishment upon the indictment of a grand jury. Which does not mean that that amendment can't be amended, it just goes to show that cruel and unusual did not in any way refer to capital punishment. Otherwise the 8th amendment would have stricken the mentioning of capital punishment from the 5th amendment.

ALSO, on a more general note, I must agree there are problems with capital punishment right now. Statistics show race as a large factor in the death penalty. In researching Mumia Abu-Jamal I learned a lot about how fucked up our capital punishment is. It's amazing how many death row inmates have been released prior to execution only because of DNA evidence available after their trial. In cases like Mumia Abu-Jamal in which there is anything BUT clear evidence of his guilt, a highly questionable judge, and a ridiculous court-provided attorney, the death penalty should not even be considered. So I'd consider the problem to be not the death penalty itself, but the horrid legal system which dishes it out unjustly.

Mumia may have done it, but the point is the trial was far from fair - if he did it, a fair re-trial would still prove it, and everyone would be happy...
Capital punishment should be reserved for only the most clear cut cases, not just beyond a reasonable doubt but beyond all doubt.

Note: The definition of capital reads "punishable with death", not "a crime involving death". The term capital has no direct indication of murder; treason is a good example of a capital offense lacking death.