Assange was blatantly targeted by a US intelligence conspiracy against him and it's blindingly obvious that Sweden wouldn't have had an arrest warrant against him without their meddling. The US also brazenly suspended the writ of habeas corpus in the GWOT and was absolutely indefinitely detaining people. The secret CIA black sites that were part of the same GWOT were also very well covered. Beyond what others have said, it's already public knowledge that the possibility of assassinating Assange was discussed by US officials.

I'm not saying this would have been Assange's fate, I would expect things to play out more like the Manning case. Still if somebody makes themselves an extremely prominent enemy of US intelligence, regardless of if you're an islamic terrorist or a whistleblowing journalist, it's very reasonable to theorise that there might be a conspiracy against them.

Usually the problem with conspiracy theories and why they're so mad is they presume thousands of actors act in concert with no clear motive to do so and all can keep a secret. This is a conspiracy which didn't really need to be kept all that secret, with a pretty blatant unifying motive - taking down Assange meant stemming leaks both directly and by making an example of him.