Painful execution of vermin may be cruel, but it should NOT be unusual. Constitution upheld. Problem solved.
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While I'd love to agree with your interpretation, the problem is that English is NOT a logical language. It seems most likely to me that the "and" in "cruel and unusual punishment" is not a logical AND at all, but a logical OR.
On the other hand, I'd love to see something - anything - that might put my interpretation to rest. Believe me, I WANT to be wrong.