I could see a strict constructionalist siding with the killer too, if the treaty/treaties said he needed to be permitted Consulate help. I wish it weren’t so, but under the Constitution, isn’t a ratified treaty the law of the land?
As I understand the law/treaty, the defendant was entitled to assistance from the embassy IF he requested it. He failed to request it. He was NOT denied assistance, so that should have been the end of it. The defendant MIGHT be able to make a case for ineffective counsel for the failure to request help from the embassy, but that would be a different argument.