Rereading the transcript from the Roper oral arguments, Kennedy foreshadowed his reckless grant of controlling authority to international law:

JUSTICE KENNEDY: Let -- let's focus on the word unusual. Forget cruel for the moment, although they're both obviously involved. We've seen very substantial demonstration that world opinion is -- is against this, at least as interpreted by the leaders of the European Union. Does that have a bearing on what's unusual? Suppose it were shown that the United States were one of the very, very few countries that executed juveniles, and that's true. Does that have a bearing on whether or not it's unusual?

For Justice Kennedy, that was apparently enough to overrule his discomfort with some of the other factors eventually cited in his opinion:

JUSTICE KENNEDY: [The American Psychological Association] came to us in Hodgson v. Minnesota, as I think the State quite correctly points out, and said that with reference to theage for determining whether the child could have an abortion without parental consent, that adults -- that they -- that they were risk -- that they could assess risk, that they had rational capacity, and they completely flip-flop in this case.

Justice Scalia made much of this in his convincing dissent, which even cites Justice O'Connor at significant length.

I find Justice Kennedy's capture by the forces of international opinion most troublesome in its implications for future terrorism cases. As I mentioned after hearing Professors Heymann and Dershowitz in Panels 5 and 6 of the Symposium, they see agreement in the subjective opinion of other nations as sufficient to create international law that's binding on the United States. To be fair, Prof. Heymann primarily articulated this as a binding canon of statutory interpretation for treaties signed by the United States. Given the number of international agreements to which the U.S. is committed in one form or another (see, e.g., the reference to the United Nations Convention on the Rights of
the Child, cited in Justice Scalia's dissent), that still leaves broad room to operate. And Prof. Dershowitz's arguments do go further, because much of his discussion of the legality of pre-emption took place in a context divorced from treaties and their signatories.