Albert Barnes Commentary 2 Kings 3:27

"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 wrath against Israel: and they departed from him, and returned to their own land." — 2 Kings 3:27 (ASV)

Compare the marginal reference. When his attack failed, Mesha, as a last resort, took his firstborn son and offered him as a burnt offering to appease the clear anger of his god Chemosh and to obtain his help against his enemies. This act was thoroughly in keeping with Moabite customs.

And there was great indignation against Israel - This means that either the Israelites were indignant with themselves, or the men of Judah and the Edomites were indignant with the Israelites for having caused this profane sacrifice, and the siege was abandoned.