[akhtene]

However crazy it may sound but it seems to me Saruman knew quite well what he was doing while accusing Wormtongue of cannibalism, even if that wasn't true. Thus he made Wormtongue look the meanest of all creatures beyond redemption and pardon. And the cornered rat, you know, fights back most fierously.
I see this scene as Saruman's suiside. His fight was lost. He couldn't ask for mercy (was too proud for that and too used to dominating others). He wouldn't be ever accepted back to Valinor after his treachery. I don't think he was willing to spend the rest of his eternal life in M-E in the shape of an old man despised by everyone.
But getting a wretched creature to kill him, or better say his body, and making all the good guys feel guilty and full of foreboding after his 'prophesy' - that was his style to get out of the situation with all his pride and wits.