What is the most appalling thing in the Bible?

That is a subjective question, what I would say is there are some instances that are a bit exceptional on their face. I submit the following accounts, although these are not all inclusive throughout scripture, but these would be some examples to answer your question.

David purchases a wife by killing two hundred men and takes their foreskins and presents them to the woman’s father a payment.

“Wherefore David arose and went, he and his men, and slew of the Philistines two hundred men; and David brought their foreskins, and they gave them in full tale to the king, that he might be the king's son in law. And Saul gave him Michal his daughter to wife.” 1 Samuel 18:27

Men rape another mans concubine and the man whom she belonged to, cuts her into 12 pieces and spread her body throughout Israel.

“And when he was come into his house, he took a knife, and laid hold on his concubine, and divided her, together with her bones, into twelve pieces, and sent her into all the coasts of Israel.” Judges 19:20-29

A disproportionate account, some children mock Elisha and he curses them and two bears kill 42 kids.

“And he turned back, and looked on them, and cursed them in the name of the Lord. And there came forth two she bears out of the wood, and tare forty and two children of them.” 2 Kings 2:22-25

God Prevents anyone from giving sacrifices who have blemishes

God tells Moses that he is very particular about who can offer food at his temples. No one with a "defect" is allowed. That is, "no man who is blind or lame, disfigured or deformed; no man with a crippled foot or hand, or who is a hunchback or a dwarf, or who has any eye defect, or who has festering or running sores, or damaged testicles". Leviticus 21:17-24

Ehud, an assassin, stabs a king called Eglon. Eglon is extremely fat, so the whole sword, up to the handle, is lost within his rolls of flab. Eglon then defecates on himself and dies, and Ehud leaves without his sword. Judges 3:12-26

God tells Ezekiel that in memorial of the siege of Jerusalem, he needs to build a model of the city. Then he needs to lie down on his left side for 390 days, then his right side for 40 days. During this time he is only allowed to eat bread which he has baked over a fire of human dung. Ezekiel protests at this last bit, so God lets him use cow dung instead. Ezekiel 4:1–17

Lot and his two daughters fled Sodom, and are hiding in a cave. His daughters worry that there aren't any men around. So they take it in turns to get their father drunk and have sex with him. They both get pregnant, and their sons each start great dynasties. Genesis 19:30-38

A man called Balaam beats his donkey for stopping. God gives the donkey the power of speech, and the donkey complains about being beaten. Balaam is apparently unfazed by the talking donkey and threatens to kill it. Then an angel comes along and tells him that the donkey saved his life. Numbers 22:28–33

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