When they read 'cruel and unusual' punishment all they read is 'cruel.'
The 'usual' ways work just fine, (hanging, firing squad, etc...). Death for a criminal isn't supposed to be 'painless' only quick, 100% efficient and VERY public.
These absurd medical 'put down the dog' procedures that sometimes even fail, (causing incredible pain and horror), are exactly what our founders meant by 'unusual.' When they fail, they are also unmistakably 'cruel' as well.
Our founders hung convicted civilian murderers on the day after they were convicted by a jury of their peers, unless that day fell on a Sunday. Then it was done at the stroke of midnight on Monday. Military criminals convicted of capital murder by tribunal were shot by a firing squad according to the same schedule. This is the model they left us and their conscience was just fine with it. So is mine.