Question: Why was Hashem angry at Sara's laughter, not at Avraham's? (Yes, I know there many answers to this, but here's another.) Question: What on earth does the haftara have to do with the parasha? (Yes, I know, but here's another.) Question: Why does the kid's mother keep saying everything's okay despite the fact that he's dead?
Answer: There's a concept called al tiftach pe lasatan (don't open your mouth to Satan, the accuser), as we shall see. Avraham said, to himself (bilibo, chap. 17 verse 17) that he couldn't have kids, whereas sara said it aloud (lemor, chap. 18 verse 12); once it's pronounced, brought into the world, made tangible (so to speak), it's fact. It's true. So Hashem Himself had to return (chap. 18 verse 14), not just the angel (cf. chap. 18 verse 10) to rectify. That's why, in the haftara, she didn't say aloud what was wrong. [This dvar Torah, until here, I heard from Rabbi Moshe Scheinerman in the name of Mrs. Aryeh Levin. The rest is mine.] A related concept is "en hab'racha m'tzuya ela b'davar hanisma min haayin," which explains the connection to the first part of the haftara: she locked the door.