Any criminal action committed by him was committed while he was in the UK. That's where any trial should take place, and if the UK authorities don't want a trial, because they know they couldn't get a conviction, that should be the end of the matter

Insofar as he exposed failings in the security of US government computer systems, he has done the US authorities an enormous favour. So it also caused embarrassment, and required them to spend money patching up the system? It will have served to make it harder for cyber-terrorists in the future, which is a lopt more important.

The ungrateful bastards should be thanking him, not pursuing him, very likely to his death.