There is a good deal of talk in Herzog about madness. The protagonist himself, though claiming to be the veteran of a marriage to an insane wife, is accused throughout the novel of having gone off his rocker. Considering that we are locked inside Herzog's skull throughout the entirety of the novel, what gives us reason to believe that the narrator is not himself completely barking mad?
There is certainly evidence to support this. When Herzog struggles to extrapolate sympathy from figures, such as Dr. Edvig and Sandor Himmelstein, who might understand both sides of the equation, he is confronted with implications of his own mental instability. While any level-headed person might be shocked at such an accusation, Herzog is furious. He immediately assumes his advisors are "siding" with Madeleine, that they have "put her up to it." He isolates himself from possible benefactors in his paranoia that they will turn or have already turned on him.
In addition, the conversations recounted seem unreal at times. It is unlikely that Sandor Himmelstein, knowing personally the gravity of Moses' plight, would consistently harp on Moses about Madeleine's superior attractiveness or prospective lovers. It is unlikely he would even become so impassioned. The cruelty perceived in Sandor and in others could very well be the results of demented exaggerations.
Furthermore, Madeleine clearly seems to be enduring some troubles of her own; we as readers have no evidence that Herzog did not in fact provoke her. It is quite possible that he forced himself on her, thinking nothing of it. Herzog demonstrates throughout the prose a remarkable ability to repress memories. Consider the revelatory manner in which he recalls his mother's death.
This blindness, this affinity for ignorance, gives one cause to think that Herzog is not the most reliable of narrators. Hence it is reasonable to read Bellow's novel as as much a document of insanity as a story of healing. The apparently happy ending becomes somewhat less rosy as a result.