Why is Isaiah coming down on the nation of Israel in Isaiah 1?

Isaiah rebukes the nation of Israel because of all their sin and wickedness. Jerusalem, once a faithful and righteous city where the people obeyed the Mosaic Law, had become a harlot, full of murderers. The city and nation had turned from righteousness to corruption.

Les explains that Isaiah is coming down on the nation of Israel because of all their sin and wickedness. In Isaiah 1:21, Jerusalem is described as a faithful city that had become a harlot. It was once full of judgment and righteousness, but now it was full of murderers. At one time the Jewish people were obedient to the Mosaic Law and the system, but they had become so wicked that Isaiah was castigating the nation. Les also notes that the word translated 'corrupt' in 2 Corinthians 2:17, drawn from Isaiah 1:22, conveyed the idea of a huckster selling an adulterated product, illustrating their spiritual corruption.

Isaiah 1:21

How is the faithful city become an harlot! it was full of judgment; righteousness lodged in it; but now murderers.

What city is Isaiah 1:21 talking about, and what was it like before?

Isaiah 1:21 is speaking about Jerusalem. Before its decline, it was a righteous city, full of judgment, with righteousness lodged in it, and the Jewish people were obedient to the Mosaic Law and its system. Later it became a harlot and was full of murderers.