Why does Miss Maudie lose her temper at the missionary meeting?
The women of the missionary circle are hypocrites. They purport to be good Christian women. Mrs. Merriweather goes on and on about how she and the other women support the work of J. Grimes Everett. Everett is evidently working to convert a tribe of "Mrunas." In other words, he is doing the "Lord's work" by converting the dark-skinned people of the world. Mrs. Merriweather proclaims that she supports this work to help people like this, in her mind, the less fortunate. These are people that she thinks are living in sin and squalor because they don't live they way she does and do not follow the same religion.
Then she says that, in her generosity, she can forgive and forget. Scout understands that Mrs. Merriweather also means a family of children in Maycomb. Scout assumes, logically, that she must mean that Mayella needs forgiveness. But Mrs. Merriweather replies that it is Tom's wife who needs forgiveness. Mrs. Merriweather thinks that Tom is guilty and that the best thing to do is forgive the entire black community. It's as if to say, even it Tom was guilty, the entire black community is somehow at fault. She is annoyed that any African-American in Maycomb would be upset about Tom's conviction. Her servant, Sophy, has been sulking and Mrs. Merriweather can not stand it. Miss Maudie is appalled at Mrs. Merriweather's hypocrisy. She (Mrs. Merriweather) supports the Mrunas, thousands of miles away, but she will not extend any understanding to the black community in her own town, even her own employee. Mrs. Merriweather is clearly racist. After Miss Maudie loses her temper, Mrs. Merriweather says of whites and blacks, "At least we don’t have the deceit to say to ‘em yes you’re as good as we are but stay away from us."