@Why doesn't the US get some evidence to support their allegations
First off, it's: retroACTIVE (ex post facto), not retroSPECTIVE, and the U.S. can't do that with laws. They can only have any effect from the point of establishment onward. I don't know how things work in the U.K., but whatever he could have been charged with when he was detained is all he can be charged with, now ... same penalties, same classification.
Second, the United States is not a Democracy, it's a Republic.
Um, back to the question of evidence ... let's see ... because the U.S. doesn't need any?
Correct me if I'm wrong, but McKinnon admitted his guilt, right? Therefore there is no need for a trial, at all. The guy is verifiably guilty. No evidence required.
There are only two things to be decided:
1) Which country gets to pick his punishment, and
2) Which country gets to effect said punishment
These wouldn't even be an issue if YOUR government hadn't already agreed to some form of extradition arrangement with the U.S. Bitch at your representatives, if you don't like it. See, that's what "extradition" is all about. And we don't have an extradition agreement with China, so if it's a Chinese hacker, they get to be dealt with under Chinese law with no argument from us. If your country wasn't such a blissful utopia for criminals of all kinds, we wouldn't even care.
And there's no need for torture, in this case. We only use that on people who haven't confessed enough or who we figure won't confess unless we waterboard them. And then, we only waterboard them a little bit, so it's not like old school thumb-screw torture. Didn't you read the (now-disgraced) Justice Department's memo? Jeez ... and the rumors were that the British are such courageous folk. Wusses.