why do her words and actions seem "purely horrible" to him?
Fifteen-year-old Vera has mischievously convinced Framton Nuttel that her aunt Mrs. Sappleton lost her mind when her husband and her two young brothers died by being engulfed in a bog while out hunting, and that her aunt leaves the window open every evening because she expects them to return through that big French window in time for tea.
When Mrs. Sappleton appears she begins talking about the supposedly dead men.
"I hope you don't mind the open window," said Mrs. Sappleton briskly; "my husband and brothers will be home directly from shooting, and they always come in this way."
She continues talking about the missing men and about hunting.
To Framton it was all purely horrible.
He naturally assumes that the poor woman is insane Both Vera and her aunt, in separate ways, have Framton convinced that the three hunters are dead. Therefore, when they appear outside, heading for the open window, he experiences the illusion that he is seeing three ghosts. He turns to look at Vera and sees that the girl, who has been described as extremely self-possessed, is "staring out through the open window with dazed horror in her eyes." She is putting on an act, of course.
This is enough to make poor Framton go running out of the house without apology or explanation..