Actually, it WAS required for capital sin (murder).
Vis. Gen 9:6 (if memory serves) "If a man sheds another man's blood, by MAN shall his blood be shed."
God is giving Noah's descendents the rules that they are to follow in the governing of their society. In effect, God says that if you don't have capital punishment, you don't have a valid nation.
I take the case of David murdering Uriah in order to steal his wife to be cold-blooded murder. For that murder, David was NOT put to death.
Therefore, even though the letter might have prescribed it, the actual practice did not require it.