There are many well known US members of congress and senate who promised to bring him to death. That was one of Assange lawyers arguments, until they all started dying suddenly.
The relevant US law with the death penalty is the Espionage Act from 1917, Section 704. It prohibits anyone from obtaining national defense–related information and making national defense–related information available to a foreign government.
Due to this the UK is disallow to extradite him, because that are the exact allegations prominent US politicians and lawmakers accuse him of.
