Here's a cynical interpretation: Hong Kong makes zero sense given that they are a) within Chinese sphere of influence and b) frequently extradites to the US. He's go other sensitive documents he may sell to the Chinese in exchange for money and sanctuary. So maybe that was the plan all along. Everyone focuses on the NSA wire-tapping scandal (which, honestly, I think everyone assumed was happening for years), and he gets a payout and some cover.
I don't think it makes sense. Like he said in his interview, he could have just sold the stuff he already leaked. Why then would he bother with the whole trouble of very publicly becoming America's Most Wanted? He sounded pretty sincere about doing this for the good of the American people. He claims he made a concerted effort to not reveal any information that could harm individuals, which appears to hold up.
I would think (hope) he has some cards up his sleeve to protect himself, but at this point I think all the double-agent/working with China talk is wild conjecture.
Don't get me wrong, I have nothing to go on. Edward's disclosure was responsible, and certainly served a public good, and so far he should be lauded for it. It was a brave act. I understand him fleeing and seeking asylum in another country given that there is little chance he would get a 'fair' trial here (as has been shown by the way Bradley Manning and Aaron Swartz have been treated)... but of all places ... China??!? He's not an idiot, so he must have thought this through but it makes no sense.
It makes all the sense in the world. Who else would benefit from keeping America's crimes in the public light, by granting asylum to one of its citizens with the courage to reveal those crimes to the world?
HK and US have a mutual extradition treaty for actions that are illegal in both countries. Presumably leaking state secrets is illegal in HK. Furthermore, HK is quite willing to extradite to US and does so on regular basis.
So the question of why China (or HK if you prefer), remains.
Too cynical?