Radak: First I will pay (punish) for profaning the land, which is a Tahor land, and they were Metamei it. The land does not bear abominations and evil deeds, like it says "Elohei Nechar ha'Aretz" (Devarim 31:16), and "v'Lo Saki ha'Aretz Eschem b'Tama'achem Osah Ka'asher Ka'ah Es ha'Goy Asher Lifneichem" (Vayikra 18:28). Even though I did not command them and warn them about abominations, just like I did not command them and warn the nations that were in the land beforehand, even so, the land did not tolerate the abominations, and vomited them out. Even though the Goyim were not command, Hashem gave to them intellect and understanding. Based on intellect, one should avoid all abominations, and from serving other gods of wood and stone. All the more so Yisrael, who were commanded, it is proper that the land vomit them out. Therefore, first He will punish them for profaning the land.

Malbim: First I will punish for their deeds, and not for the ways and Midos hidden deep in the Nefesh, for their sin was double. Refer to 16:18:5:1, 16:18:3:1.

Rashi: They repeated their sins, to do the sins of their fathers. Radak - "Poked Avon Avos Al Banim" (Shemos 20:5), when they persist in the [evil] deeds of their fathers, like it says there "l'Son'ai".

Rashi and Radak, both citing Targum Yonasan: I will pay (punish) for the latter like for the former, two for one, because they profaned the land, the place of My Shechinah.

Radak: This is the level of their disgusting acts and abominations. Like this, it says "v'Hishlachti Alecha Shikutzim v'Niblatich" (Nachum 3:6). Serving Shikutz (idolatry) is repulsive and despised.

Malbim: (They sinned against the Beis ha'Mikdash;) Menasheh set up an idol in the Heichal.

Radak: It is the land proper for Me to make My Shechinah rest in it. Also above (2:7) it says "va'Tavo'u va'Tetam'u Es Artzi v'Nachalasi Samtem l'So'evah." It is more Kadosh than other lands, for a reason known to those who understand knowledge.

Malbim: They did not guard Shemitah and other Mitzvos dependent on the land. Also, they were Metamei the land through their deeds - "Ki ha'Dam Yachanif Es ha'Aretz" (Bamidbar 35:33), "v'Lo Saki ha'Aretz Eschem b'Tama'achem Osah" (Vayikra 18:28).