21:26a He committed the most detestable acts by going after idols,

Ahab acted in a very disgusting/terrible way. He worshiped idols,
-or-
He served idols/carvings, which is very offensive ⌊to God/Yahweh⌋.

21:26b just like the Amorites whom the LORD had driven out before the Israelites.)

just as the Amorite people had done. Yahweh had driven/pushed out the Amorites when ⌊the people of⌋ Israel came ⌊into the land⌋.
-or-
That is what the Amorite people had done before Yahweh sent them away ⌊from the land⌋ so the Israelites could enter/live in it.

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