If you look at the introduction of The Glass Menagerie, Tom explains to us that he is the narrator of the play, and that he has the soul and mentality of a poet, which is the reason why he gives symbolic and special characteristics to several aspects of his story: He even gives such unique traits to himself.
TOM: [.......] He is the most realistic character in the play, being an emissary from a world of reality that we were somehow set apart from. But since I have a poet’s weakness for symbols, I am using this character also as a symbol; he is the long delayed but always expected something that we live for.
Tom describes himself as "an emissary from a world of reality". This means that he is more than willing to recognize the flaws of his own character. He is not forgiving of the flaws of his mother, nor his sister. He simply tells the story just the way it is. Like he says, he is merely giving special characteristics to the story, in order to, in his own words, be able to remain in that world in which the Wingfields "where somehow set apart from".