Snowden's method of whistleblowing was leaking documents to the press. He is both.
If you killed someone committing their own separate crime, you'd be both a killer and potentially a hero.
It is important to make clear he both committed a very serious crime and potentially blew the whistle in act of bravery.
Using "leaker" means you are ignoring that what he did was to blow whistle on illegal activities and only focusing on him.