I think the phrasing cruel and unusual was intended to allow the standard to change over time, as social mores changed. While hanging in the public square, or even at the sight of capture was once permitted in the wild west, we now find a public hanging to be unacceptable to public sentiments, even in a state where capital punishment is upheld by popular vote.
I do not think capital punishment is cruel if it is administered to an adult convicted of crimes that include preying on weaker people. A classic case would be an adult killing a child after raping the child, for the purpose of the child being unable to testify against the adult. I have no problem with capital punishment in that case and many other cases. I do have a problem with the death penalty for all people convicted of murder. There are many cases where the act was not technically self defense, but a strong moral argument is made for the act being understandable given the circumstances and justice being available with a much lighter sentence. An example might be a young adult robbing a store due to a pressing need for his child or family member. The store owner pulls out a gun and fires a shot at the robber but misses. The robber returns fire and kills the owner. A crime yes, unacceptable behavior. But death as the sentence is going too far, in my opinion.
I also think that it is unreasonable for society to be expected to house a young adult for his entire life in a high security facility because he has proven to be a danger to society through several separate actions, such as rape or severe physical assault. I think society has a right to conclude after enough chances, that a criminal is a hazard to society and is not going to reform, and that criminal should be eligible for the death penalty simply to protect society. In other words, life in prison with no chance for parole is an inappropriate sentence for many criminals. If there is no chance of the person being freed, then capital punishment may be appropriate in many, but not all cases.