SCOTUS has ruled that the death penalty _is_ cruel and unusual punishment in many circumstances.

For example, in 1980 they ruled that it could only be applied only if it involves a precise and aggravating factor[0].

Louisiana had a law that made the rape of a child punishable by death and SCOTUS overturned that law in 2008[1].

SCOTUS has consistent narrowed the crimes punishable by death in the past 50 years using the Eighth Amendment as a justification.

[0] https://en.wikipedia.org/wiki/Capital_punishment_in_the_Unit...

[1] https://en.wikipedia.org/wiki/Kennedy_v._Louisiana