Malbim: (a) Achaz was not as evil. He sealed (stopped study of) Torah; Menasheh erased the names of Hashem from the Torah (Sanhedrin 103b). Achaz did not intend to anger, only to cast off the yoke. Menasheh intended to anger. This is why it says that Achaz was ashamed in front of Yeshayah (104a). He sinned only amidst desire. Menasheh killed Yeshayah! (b) Achaz did not veer Yisrael [to be Resha'im]. Not all followed his ways. Therefore it says (103b) that he erected [an idol] in the upper story of the Mikdash. He could not put it in the Heichal, for the nation was not drawn after him. Menasheh made Yisrael sin with his idolatry (so he was able to put an idol in the Heichal).