[sneak]

Because we as a society have generally reached a consensus that premediated killing of human beings is one of the worst crimes possible.

The fact that it is regarded by some as an acceptable punitive measure carried out by the state, whereas other, lesser crimes against human beings are deemed "off limits" for such purposes, seems to me to expose an inherent contradiction. To avoid the contradiction, either both should be acceptable, or neither.

Presently there seems to be a slight majority consensus on Earth that the correct answer is "neither", with the overall trend suggesting that the rest of the planet will get there in another couple hundred years.