What I did find in my search of which I was not aware, is that "the term capital comes from the Indo-European kaput, meaning "head". Thus, capital punishment is the penalty for a crime so severe that it deserves decapitation (losing one's head).

Also somethings of which I was also unaware, 90% of world's state sanctioned capital punishment occurs in four countries. The People's Republic of China, Iran, Saudi Arabia and the United States and surprizingly, The United Nations Convention on the Rights of the Child, which forbids capital punishment for juveniles, has been signed by all countries except the USA and Somalia.