It wouldn't have to be the US or even the UK. Assange has angered a lot of different governments, not to mention private citizens.
People die in London police custody at about the rate of one every month, if I remember correctly. It's not really such an unusual occurrence. Without knowing much about the UK, I doubt that the police officers who kill those captives usually get indicted for murder.
And I doubt even the US would be stupid enough to try something like that even though there are enough politicians baying for blood.
I wished they were as assertive about bringing people to trial when it comes to the Guantanamo inmates.