God got involved in this killing because he liked Asa, the king of Juda, so much. Asa did what was good and right in the eyes of the Lord by destroying the temples of other gods and forbidding their worship.
Asa did that which was good and right in the eyes of the LORD his God: For he took away the altars of the strange gods, and the high places, and brake down the images, and cut down the groves. 2 Chronicles 14.2-3And he drove homosexuals out of the land.
And he took away the sodomites out of the land. 1 Kings 15.12So when the Ethiopians attacked Judah while Asa was king, you know which side God was on.
There came out against them Zerah the Ethiopian with an host of a thousand thousand. 2 Chronicles 14.9All Asa had to do was ask.
Asa cried unto the LORD his God, and said, LORD, it is nothing with thee to help, whether with many, or with them that have no power: help us, O LORD our God; for we rest on thee, and in thy name we go against this multitude. 2 Chronicles 14.11And God killed all 1,000,000 Ethiopians, apparently all at once, in a single day.
So the LORD smote the Ethiopians ... they were destroyed before the LORD. 2 Chronicles 14.12-13It was the largest, single, God-assisted slaughter in the Bible. (And according to McEvedy and Jones, it would have killed five times the entire Ethiopian population at the time.)