You are correct that the traitors who passed these secrets on to Assange, and those further up the chain of command who allowed it to happen, and possibly those in the White House and elsewhere who arranged for it to happen, should be punished for their treason.
But Assange has shown himself to be an enemy to our country, who has deliberately set out to damage our country and its relations with others, as well as to cause the deaths of informers and others whose identities have been revealed.
It's like the difference between a foreign spymaster and the domestic traitors who are willing to work for that spymaster. Both deserve fit punishment for their crimes.
The usual penalty for such crimes is death.
yes, Assange is an enemy of the USA and is doing his damnedest to undermine military, diplomatic, and counter-terrorism operations worldwide.
The fact that as a non-citizen he cannot be charged with “treason” does not mean he should not be regarded as a very dangerous spy committing espionage against legitimate national security interests of the USA (and many other countries).