Why does it say "v'Hemah Lakchu"?
Malbim: They did not consider this theft, rather, like [spoils of] war, for Michah chased them and could not overpower them.
Why did these idolaters succeed?
Radak and Malbim, both citing Tanchuma Ki Sisa 14: "Lecha Hashem ha'Tzedakah v'Lanu Boshes ha'Panim" (Daniel 9:7).
Why did they burn the city? They wanted to settle there! The only intact city that Yehoshua burned was Chatzor, because it headed the kingdom, and this confounded everyone!
Radak #1: They entered the city suddenly and set fire to the city to confound the residents, and they would not be able to fight back amidst the fire and the war of Bnei Dan.
Radak #2: Perhaps the city was full of idols. Even though they set up their image (verse 30), they intended l'Shem Shamayim. Even so, it was a sin - "Lo Sa'asun Iti Elohei Chesef..." (Shemos 20:20) - even if you intend to serve Me.
Emes l'Yakov (Bereishis 14:14): Initially the city was called Dan, the name of an idolatry (refer to 18:29:1:2). At the time of Matan Torah, it was changed to Layish. Perhaps also this is the name of an idolatry, in the shape of a lion; Layish is one of the names of a lion. They wanted to rename it Dan, like their ancestor. They burned it, lest people say that it returned to its initial name (of idolatry).
