At that point, you have to try to draw some kind of distinguishing, which will take quite a lot of time, money, and effort, just to create a punishment that is barely ever used and accomplishes no meaningful advantage over life imprisonment except some sort of rather perverse moral satisfaction.
In a perfect world, I'd agree that death for the most heinous case that have no legal ambiguity is essentially fine, but in reality, "not legally ambiguous" functionally does not exist, or at the least, it takes a lot of time and money to find it.