Doubt it, if the US wanted him dead, he'd be dead already. After all, even Russia can kill people, usually spies, all the time in Britain and other Western countries without major consequences, although I guess Assange might be a little too prominent for this to work well.

Similar to Navalny, he'll be made an example of by spending the rest of his life in prison, maybe a supermax if they're feeling particularly cruel. The US doesn't want to turn him into (more of) a martyr.