While it may seem ironic due to Miss Strangeworth's hurtful behavior, both righteousness and purity are values she regards with utmost importance.
Even though the things she writes to her neighbors can be hurtful and mean, she only says them because she wants to save them from any "evil" that may be lurking about.
For example, she writes a letter to Mr. Lewis alerting him that his grandson is stealing from the cash register at work. Similarly, she writes a somewhat vague letter to Mrs. Harper asking her if she has yet discovered what the other women at the bridge club were laughing about once she left. She signs off questioning whether the wife is actually the last to know, implying that her husband is cheating on her.
While Miss Strangeworth usually has no definitive proof when she writes her letters, she feels justified in sending them anyway because she believes that what she's doing is good.
When describing Miss Strangeworth's attitude towards her "work," the narrator says that while the letters may be harsh, "wickedness was never easily banished, and a clean heart was a scoured heart..." (Jackson, 6).