There is simply no evidence that anyone in "the national security bureaucracy" is entertaining the idea of killing Julian Assange. There isn't even any evidence that any military or law enforcement agency in the US is entertaining the idea of apprehending him. On what grounds would they do so?
It is, on the other hand, very easy for me to accept that the US would try to recruit any one of the tens of thousands of well-armed, morally ambiguous bandits carousing around the western tribal provinces of Pakistan. After all, tens of thousands of well-armed American and NATO fighters are waiting just on the other side of an invisible border between Afghanistan and Pakistan to shoot at those same bandits.
There is simply a world of difference between the Assange scenario and the AfPak scenario; an avalanche of incentive exists in AfPak, and yet we're still bound by a kabuki dance of "rules of engagement" and "surgical" airstrikes.
There's no way I can win an argument about this with you, because it demands that I prove a negative: that nobody in the US government, no matter how ineffectual or mentally incompetant, would, given the chance, seek to neutralize Julian Assange (by PR, by arrest, or by murder). But suffice it to say: you're talking to someone who believes that if agents from the FBI, CIA, and AFOSI were standing right in front of Julian Assange in an unlit alley blocked on either end with vans ready to spirit him away, they'd try to get him to answer a few (or many) questions... and then leave him to walk away.