[0] https://abcnews.go.com/Politics/us-gave-verbal-pledge-death-...
[1] https://www.cnn.com/2017/01/04/politics/kfile-trump-wikileak...
Well ok, they technically can break their own treaties but then no EU nation will ever extradite to them again. And many other countries require similar assurances which would now be worthless.
Incidentally, the maximum penalty for the charges is five years, which is far short of a death sentence.
> Extradition is prohibited by statute if:
> ...
> there are no speciality arrangements with the requesting country – ‘speciality’ requires that the person must be dealt with in the requesting state only for the offences for which they have been extradited (except in certain limited circumstances)
Here's the relevant law: https://www.legislation.gov.uk/ukpga/2003/41/section/95
However it looks like the US indictment[0] is more or less the same as the formal charges will be, considering the exact same maximum sentence as for the indictment was stated at the UK hearing (five years).
[0] https://www.justice.gov/usao-edva/press-release/file/1153481...