I think that when we think of George Sanders we tend to see him doing some dirty business. He's cheating his nephew Tyrone Power out of a noble title and estate (SON OF FURY), or he's leading a pack of Nazi agents in London to track down Walter Pidgeon and Joan Bennett (MANHUNT), or he kills poor Nelson Eddy in a hopeless sword duel (BITTERSWEET). But there was always something more in his characters (even his villains). Take his Oscar performance (Addison De Witt in ALL ABOUT EVE). Yes, he puts his powers to the benefit of Eve Harrington, with an eye to her being his permanent partner (I don't think marriage is necessary - he doesn't look like he'd like a family, or domestic arrangement). But if you follow Joseph Mankiewicz's dialogue carefully, Addison is more complex and acceptable than Eve. He is in her corner because as a theatre critic he realizes that she is talented, and can bring back a youthful vigor to the parts that Margo Channing is constantly playing. Look at the scene where he tells Margo and Karen what a wonder Eve's performance is when she reads for the understudy position. It's not just sexual allure, but he really likes her talent. Moreover, Addison is a realist about himself and theatre people in general. He admits he has limitations (he's not a people person), but he does love good art. Actually, in some respects he is a better person than he admits. Karen goes to speak to him (we are told by Addison in his famous scene with Eve in the Hartford hotel) to find out what he knows about her husband and Eve. While he makes a snide comment about Karen telling more than learning, it's obvious Karen does consider Addison a friend - even an ally. And actually, by putting Eve into her place finally (Addison appears to be the only one with brains who could) he does save Karen's marriage as well.
It comes as a pleasant surprise to movie goers that Sanders could (given his talent and a good script) appear as a nice guy. He does so in this film. Uncle Harry is a decent man who lives with two sisters, and who keeps the family household going. The younger of the sisters, played by Geraldine Fitzgerald, is too attached to him - and for a 1940s film the clutching of Ms Fitzgerald spells out incest more than was usual (interesting to think that this 1945 film comes only two years after Hitchcock's SHADOW OF A DOUBT, where "Uncle" and "Niece" Charlie are very close - until Theresa Wright begins to wonder how Joseph Cotton makes his living). The arrival of Ella Raines as a love rival sparks all out warfare from Fitzgerald, with Sanders befuddled about which way to turn. Raines seems to leave town, and Fitzgerald seems victorious and Sanders is morose when he finds a bottle of poison in the house, and begins to reconsider his options.
It is hard to see now what ending could have been tacked on to the film to make it satisfactory to everyone in the audience. The moral code of the 1940s made it imperative that if a villain kills someone, no matter how lively, likeable, or sympathetic the villain was he or she had to pay. Fitzgerald could only pay if she were defeated by Raines. If Sanders died that would not have defeated Fitzgerald. If Sanders lived in despair after Fitzgerald's death that would not help either. I think the film's "trick" ending here is as good as it could be. But that is only my opinion.