Adam Clarke on Micah 1:13

Inhabitant of Lachish - This city was in the tribe of Judah, Jos 15:39, and was taken by Sennacherib when he was coming against Jerusalem, 2Kgs 18:13, etc., and it is supposed that he wished to reduce this city first, that, possessing it, he might prevent Hezekiah's receiving any help from Egypt.

She is the beginning of the sin - This seems to intimate that Lachish was the first city in Judah which received the idolatrous worship of Israel.