The exact words “cruel or unusual punishment” were first used in the English Bill of Rights 1689. The Founding Generation understood the following as cruel and unusual: pillorying (the stocks), accompanied by corporal punishments such as flagellation (the "whipping post") or mutilation such as branding or having an ear cut off (cropping); disemboweling; decapitation; and drawing and quartering. The common denominator: the causing of prolonged pain to a prisoner who is physically restrained, i.e. torture.
“Disproportionate” or “excessive” punishments first arose as legal concepts in dissent in 1892. They were adopted in 1910. In 1954, Chief Justice Earl Warren fully rejected the original understanding in favor of the "evolving standards of decency that mark the progress of a maturing society." Ever since, the Supreme Court's view of cruel and unusual has been completely untethered to the electorate.
For perspective, the Governments of China, Iran, North Korea, Saudi Arabia, Somalia, Hamas-controlled Gaza, and the Islamic State of Iraq and the Levant conduct public executions, often in mass, to demonstrate state power to their populations. Decapitation is used in Saudi Arabia and Qatar. Executions continue for adultery and apostasy in Saudi Arabia and Iran, blasphemy in Saudi Arabia and Pakistan, sorcery in Saudi Arabia, and "economic crimes" in China. In 2011, at least 32 countries impose capital punishment for drug offenses.

Capital punishment, as practiced in the United States of America since 1791, is not what we are talking about here.
Torture, as the Framers knew it, is dangerous to the nation as a whole. The sociopaths required to conduct such heinous acts will be chosen for their obedience to authority. Governments that train and employ state agents practiced in torture are a risk to your life. To wit: China, Iran, North Korea, Saudi Arabia, Somalia, Hamas-controlled Gaza, and the Islamic State of Iraq and the Levant.
Insofar as the “Living Constitution” fallacy is concerned, it now seems no criminal penalty shall be imposed which the Supreme Court deems unacceptable. The cruel and unusual punishments clause is completely arbitrary today. This is also dangerous. Future Justices, appointed by regimes less concerned about, well, you - could look the other away during a national emergency. Call it the devolving standards of decency that mark the regression of a collapsing society.