Rashi: If breakage and Ones came to them, they caused it to themselves - "Ach Shav Hayu..."

Radak: If Gil'ad began to do evil, they were exiled first.

Malbim: I explained (6:8) that all the Nevi'im, Talmidim of Eliyahu, were killed in Gil'ad, where Eliyahu was. There Izevel killed Nevi'ei Hashem. About this, it says "Gil'ad Kiryas Po'alei Aven Akubah mi'Dam" (6:8). Our verse teaches, if Gil'ad is full of sin to kill the Nevi'im, these Nevi'im were for nothing (did not help at all).

Radak #1: This refers to above (Gil'ad); the matter is repeated in different words.

Radak #2: This refers to below (Gilgal). Even though the note on "Hayu" is Zakef (a stop, i.e. it is not connected to what follows), the Perush does not always follow the notes.

Radak: Bnei Gilgal were neighbors of Bnei Gil'ad; only the Yarden separated them. They learned from their deeds, and began to serve idolatry like them. They slaughtered bulls to idolatry where they made a Mizbe'ach to Hashem and erected the Mishkan initially after crossing the Yarden.

Malbim: Like the number of Nevi'im that they killed, they slaughtered bulls in Gilgal to idolatry and erected Mizbechos to Ba'al - a Mizbe'ach and bull for every Navi that they killed.

Sanhedrin 102b: In every furrow in Eretz Yisrael, Achav erected an idol and bowed to it.

Rashi: They are numerous like mounds on the furrows of a field.

Malbim: They thought that via the Mizbechos, they atone for the Nevi'im that they killed.

Radak: Not only did they make a Mizbe'ach to idolatry - they also built many Mizbechos outside the city. They appeared like mounds of rocks on furrows.

Radak: This is like Sadim. Also "v'Kara Lo Chalonai" (Yirmeyah 22:14, is like Chalonim).