It's a strange phrase -- "impolitic cruelty." Cruelty badly timed? Cruelty which disregards the feelings and best interests of all parties? Probably, the latter. But is there a politic cruelty? Can cruelty be in the best interests of people, of anyone? All cruelty, you would think, disregards feelings and is oblivious of the best interests of others, even the best interests of the perpetrators (in this case, Mrs. Ferrars).