"A gentleman through and through," is how Stevens describes his former employer, Lord Darlington (2.105). Huh. Darlington became the host of English Fascists and was a Nazi sympathizer. So how can such a man be considered "a gentleman," exactly?
Lucky for you, Stevens is here to explain:
And when today one hears talk about his lordship, when one hears the sort of foolish speculations concerning his motives […] I for one will never doubt that a desire to see "justice in this world" lay at the heart of all his actions. (3.180)
Stevens wants to provide a more complex portrait of Lord Darlington than just "that Nazi guy." He wants to paint Darlington as a man who had good intentions but was led astray by manipulative diplomats. In his speeches and in conversation, Darlington represents old, traditional notions of English gentlemanliness. While other diplomats, notably the American senator, tend to be more pragmatic, Darlington champions honor, fairness, friendship, and gentlemanly conduct.
But there are definitely downsides to being such a gentlemanly gentleman:
"[…] He's a gentleman, and he fought a war with the Germans, and it's his instinct to offer generosity and friendship to a defeated foe. It's his instinct. Because he's a gentleman, a true old English gentleman. […] they've used it, manipulated it, turned something fine and noble into something else—something they can use for their own foul ends? […]" (7.174)
These ideals are certainly noble, but because Darlington isn't a professional diplomat, they are directed toward iffy political aims. His amateur-hour diplomat game has epic consequences. But he started with his heart in the right place: when he noticed the economic and social devastation wreaked by the peace treaty in Germany, he felt compelled to become involved in public affairs, to the point of supporting the efforts of Herr Ribbentrop—the German ambassador—to cement Anglo-German relations.
But Darlington just seems kind of gullible on top of his "instinct to offer generosity and friendship to a defeated for." When Darlington notices the poverty of London's East End, however, he readily accepts the explanation, offered by British Fascists associated with Sir Oswald Mosley, that the Jews are responsible… although later he revises his anti-Semitic views.
Even when he recognizes that a guest has been rude to Stevens, Darlington still defends the guest's view that ordinary men don't understand politics and hence can't make rational decisions about voting or policymaking.
Characters such as the American senator and Mr. Cardinal (Jr.) see weakness in the way Darlington clings to old-fashioned values, which they think have no place in modern politics. But as Stevens's memories bear out, the question of Darlington's guilt, and his motivations, are complicated.