The point is he never said any opinion was BASED on international whatever just that they found the Teenage Killer case "consistent" with international whatever. They didn't even need to do that since 1) teenage killers are almost never executed anyway or sentenced to death and 2) many states have no death penalty for anyone.
The Justices don't need to look abroad to make bad law (unfortunately.)
No, they don't--but Kennedy did just that--at the very least to bolster his opinion and/or to make himself feel better.
Justice Scalia seemed to think otherwise. Read his opinion.