Putting your different points together:

> piles of research showing that severity of punishment is not an effective deterrent

> not think of consequences

> Deterrence means a lot less when you don't have to worry about recidivism because the person is dead

Sounds like (in general, not talking about minors) when you execute the people who for whatever reason cannot think far enough ahead for punishment to be an effective deterrent, you eventually will be left with people who are able to do that, who will comprise a less criminal society.