Further, what he's "wanted for questioning" about isn't a crime in the United Kingdom (no, he's not been accused of "rape" in the traditional sense, he's been accused of continuing consensual intercourse after a condom broke after having agreeing to use one,) nor the US, nor most other countries on earth.
The UK Supreme Court disagrees with you, in the verdict on extradition they pretty clearly stated that they were of the opinion that The actions Assange stands accused of would constitute a crime in the UK as well.
That Sweden won't guarantee him safe passage (i.e. "We won't extradite you to the USA") you can surmise that extradition to the United States is the sole purpose of getting him to Sweden in the first place. If it wasn't, they'd have long since agreed just to end this stain on their reputation: Already most Europeans see them as a tool of the Americans. Ditto the UK. I mean, most people saw them that way before this, but this has only cemented that image in their minds.
Sweden cannot make that guarantee because extradition here is a judicial process and the government like in most civil states is forbidden from meddling in the affairs of the courts. The govenment can overrule the courts under certain circumstances such as if Assange was risking death or torture if he was extradited.
The only thing the Swedish government could do to prevent a lawful extradition would be to rescind the extradition agreement with the US...
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