You know that US had in the past kidnapped people multiple times right? And sometimes they ended in torture chambers.
Also, Chelsea Manning stupidly believed that there would be fair treatment in the US, and ended in solitary confinement for life.
Also, Denmark admitted that the US planes parked there are CIA planes that are waiting to kidnap Edward Snowden in case he go Denmark, Sweden or Norway.
What makes you think, that Assange is immune from that treatment? The CIA planes are already there! If Assange go to Sweden, the US can kidnap him very easily, Denmark+Sweden authorizing it or not, and historically they have authorized quite frequently (maybe every single time).