A common theme in Roald Dahl's short stories is that of revenge on an overbearing or insensitive partner. Mr. Foster in "The Way Up to Heaven" has been rather more than that. He has been deliberately sadistic and, as his wife has recently realized, appears to have been attempting to drive her to some sort of breakdown.
Mrs. Foster's feelings at the end of the story, therefore, are all or almost all of a positive variety. There is relief at finally being free, excited anticipation at being able to spend more time in Paris with her grandchildren, and a certain satisfaction that a bully who had been tormenting her has finally got his just desserts. If there is any conflict—pity for Mr. Foster or regret at the harshness of his punishment—it is not apparent. Having spect so much time being nervous and anxious, Mrs. Foster is now perfectly composed.