For years people have claimed that the CIA or NSA or FBI have killed people within the US.  Those claims were made based on the assumption that these all powerful agencies were fearful of some information that was going to be leaked by the individual.

I think that the ease with which Julian Assange is found by news media would lead me to believe that one of those agencies could find him and kill him if they wanted to.  It also seems that the information would harm those in government giving them a reason to kill him.

The fact that he hasn't been hurt and the government isn't even making much of an effort to do anything about him should show that all those conspiracy theories have been just so much fiction.