Rashi: She saw the evil that befell her sister for her Zenus, and she did not fear [lest the same happen to her].

Radak: She saw the Churban and Galus of Ahalah, but did not refrain from trusting in kings of nations and learning from their deeds. Her desire was worse than Ahalah's (for they should have learned from Ahalah), like Yirmeyah said (3:11) "Tzidekah Nafshah Meshuvah Yisrael mi'Bogedah Yehudah." Malbim ? this was in the days of Achaz; he bribed Melech Ashur.

Malbim: In the days of Menasheh, Melech Ashur's generals came against Menasheh. (Her lover destroyed her!)

Radak: It is from the root Zanah (Zenus); the middle letter of the root is doubled, like "Hagigi" (Tehilim 5:2) from Hagah.