Sweden has a proven track record of handing people over to the CIA for rendition, who were subsequently handed over to the Mubarak regime where they were tortured. The victims had no chance of legal recourse, as the time between being nabbed and handed over was only a few hours. Whether or not Sweden's laws allow it to do this, the fact of the matter is that Sweden has done this.
In comparison, the UK has a proven track record of standing up to the US on similar issues, given that it demanded the return of its citizens from gitmo. If extradition was requested in the UK, from past experience Assange would at least have a chance to fight it in the courts, instead of being unceremoniously handed over.
They definitely couldn't do it secretly, but why couldn't they pull it off? Sure there would be outrage, but governments have faced that before over issues much larger than this. (eg. decisions to go to war and such)
Why not? They hate the guy. Can he really bet his life that they wouldn't go "oops, it happened again."? The fact that they've done this before makes it far more likely that they will do it again. That's how track records work.
It's unlikely to happen to him now, but if he hadn't've made a fuss and drawn so much media there wouldn't be such a spotlight on them.
As someone else said on this issue: it's safe for you to say 'it wouldn't happen'. If you're wrong, nothing happens to you. You get to shrug and say 'hey, I was wrong', and then only if you're feeling magnanimous.
And even then, even if Sweden were to do it by the book, it may just be that it's easier to fight extradition in the UK than in Sweden, just like certain courts are more favourable to business in the US.
In comparison, the UK has a proven track record of standing up to the US on similar issues, given that it demanded the return of its citizens from gitmo. If extradition was requested in the UK, from past experience Assange would at least have a chance to fight it in the courts, instead of being unceremoniously handed over.