They intend, fully, to make an example out of him.
Why murder him when he's no threat to them anymore, and they can drag him through more hell for the next decade in order to show what happens when you cross the line?
I'm afraid you may have fallen for the deliberately misleading words of the supposed assurances given by the US. Here's what the Guardian[0] says of that particular claim (with my emphasis):
"and could apply, if convicted, to be transferred to a prison in Australia."
My understanding is that his application could be denied, without any recourse, by the DoJ (of whichever administration is in power at the time), and probably by the Australian government too.
Why murder him when he's no threat to them anymore, and they can drag him through more hell for the next decade in order to show what happens when you cross the line?