Then he took his eldest son that should have reigned in his stead, and offered him for a burnt offering upon the wall. And there was great indignation against Israel: and they departed from him, and returned to their own land. Took his eldest son - The rabbins account for this horrible sacrifice in the following way: -
When the king of Moab found himself so harassed, and the royal city on the point of being taken, he called a council of his servants, and asked them how it was these Israelites could perform such prodigies, and that such miracles were wrought for them? His servants answered, that it was owing to their progenitor Abraham, who, having an only son, was commanded by Jehovah to offer him in sacrifice. Abraham instantly obeyed, and offered his only son for a burnt-offering; and the Israelites being his descendants, through his merits the holy blessed God wrought such miracles in their behalf. The king of Moab answered, I also have an only son, and I will go and offer him to my God. Then he offered him for a burnt-offering upon the wall.
Upon the wall - על החמה al hachamah. Rab. Sol. Jarchi says that the letter ו vau is wanting in this word, as it should be written חומה chomah, to signify a wall; but חמה chammah signifies the sun, and this was the god of the king of Moab: "And he offered his first-born son for a burnt-offering unto the sun." This is not very solid.
There was great indignation - The Lord was displeased with them for driving things to such an extremity: or the surrounding nations held them in abomination on the account; and they were so terrified themselves at this most horrid sacrifice, that they immediately raised the siege and departed. In cases of great extremity it was customary in various heathen nations to offer human sacrifices, or to devote to the infernal gods the most precious or excellent thing or person they possessed. This was frequent among the Phoenicians, Romans, and Greeks; and it was the natural fruit of a religious system which had for the objects of its worship cruel and merciless divinities. How different the Christian system! "Wilt thou that we shall bring down fire from heaven and destroy them? Ye know not what manner of spirits ye are of; the Son of man is not come to destroy men's lives, but to save them."
Commentary on the Bible, by Adam Clarke [1831].