I'm reading this whole thread and it seems that people are missing the obvious thing Obama was doing here. His entire rationale for doing this is to guarantee American access to consulates in the case they are detained overseas. What this guy did to be put on death row is irrelevant to the discussion.
The key component isn't states' rights, the ability of the President to enforce a treaty or even international law. In a practical sense, Texas had the authority to do what they did. However, what happens when an American is imprisoned under sketchy means by a country which signed the same treaty as the US? They could summarily say that we ignored agreements to consulate access, so it's no big deal that China secretly sentenced some guy from Houston for whatever without allowing him to access the consulate to assist and inform him of his rights, advocate for him if warranted and, essentially, guarantee a US citizen is not being mistreated (think the reporters imprisoned in North Korea).
Point being, it's a pretty standard protocol for foreign nationals to allow them access to their consulate and it's a poor precedent for us to break this protocol, even for some psychopath. Nobody, including Obama, is intervening on behalf of this guy, they were concerned with the larger picture and, more importantly, American citizens. Just cross your t's and dot your i's, it's not hard.
By the way, damn you guys. I said I wasn't going to post in the Politics section because I get too involved and now you pull me back in (cue Pacino).