[The Squatter of Amon Rûdh]

There's no explicit confirmation of Saruman's words, but it isn't unlikely that Gríma should do such a thing, poor debased creature that he was. My suspicion is that Saruman had been forcing him to find his own food, possibly as a punishment for throwing away the Palantír of Orthanc, and that hunger might well have driven him to eat Lotho. I suspect, however, that Wormtongue's words were a reference to the murder itself: I doubt if he would admit to the other crime before a group of armed and angry Hobbits. The point is that whatever Wormtongue had done, Saruman had forced him to it. Normally I would regard this as a pathetic excuse, but Saruman was more than capable of controlling the thoughts of one as weak-willed as Gríma even in his diminished state.

I should perhaps point out that Gríma wouldn't technically have been a cannibal even if he had eaten Lotho, since they weren't of the same species. In a moral sense, though, the line is a very fine one indeed.