I have mixed feelings about capital punishment. I couldn’t myself pull the trigger or push the button so I don’t want others to do it for me. I’ll soon be present when our 113 year old dog is put to sleep. I think I could if I had to be the one who gave the injection but still.
http://www.constitution.now77.com/
The Constitution doesn't "allow" capital punishment, what it does is accept that it is already a legitimate form of punishment & simply put limits on how it can be used by preventing it being done in a cruel or unusual manner (8th Amendment).
Not only that, the Double Jeopardy Clause in the 5th Amendment says, "...nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb...". What does the "...or limb... phrase mean? Could it mean that the Founders consider castration to be a legit form of punishment? Nothing was added to the Constitution by accident, so it had to have a purpose for being there -- every word of it.