Since you quote Matthew Henry, I guessing you believe the things he states. Let's take a look.God did not punish Cain with death. That was a "willful murder". God did not punish Moses with death for the "willful murder" of the Egyptian. God did not punish David for the "willful murder" of Uriah the Hittite. God did not punish Paul for the "willful murder" of Christians.
Maybe making an absolute statement that God will always punish willful murderers with death isn't supported by scripture.
When one defines "ought" it turns out that they should but not that they will. Matthew Henry discusses Cain, as you know: "... Before the flood, as it should seem by the story of Cain, God took the punishment of murder into his own hands; but now he committed this judgment to men,...." The other cases that you mention of Moses, David, and Paul would have to be examined in detail, but there is no indication that these exceptions overthrow the instructions to Noah or suggest that murder should go unpunished entirely.
Furthermore, if it is correct that the punishment of murder was delegated to human beings after the flood, then it is human failure that murder has gone unpunished and a sin on the part of men, who are supposed to execute murderers. And murderers are supposed to be executed quickly. Arias herself mentioned that she should just be locked away and forgotten and if she were on death row the appeals could drag on for two or three decades, and she still might escape the death penalty. She might have had that calculated from the beginning. There is nothing speedy about these endless appeals--they are a mockery of justice. King Solomon explains what is wrong with long delays:
Ecclesiastes 8:11 (KJV) Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil.