Then is not Saruman a traitor?

But let us not darken our hearts by imagining the trial of their gentle loyalty in the Dark Tower. For the Enemy has failed – so far. Thanks to Saruman.

„Then is not Saruman a traitor?“ said Gimli.
„Indeed yes“ said Gandalf. „Doubly. And is not that strange?“

JRR Tolkien: The Lord of the Rings, ISBN 0 261 10243 5