Before the promised redemption can arrive, a period of reckoning must take place. God declares that He will exact a severe payment from the people for their open and deliberate sins. This punishment must happen first, driven primarily by the profound defilement of the land. The pure land is simply unable to tolerate abominations and evil deeds. Just as the land expels other nations who corrupt their ways, even though they were never given divine commandments, it will certainly expel the Israelites who were explicitly commanded to uphold its purity [רד״ק].
The penalty enacted upon the people is a double punishment. The primary approach among commentators is that the people suffer twice over because they repeated the sins of their ancestors, piling their own wrongdoings on top of the sins of previous generations. Alternatively, [מלבי״ם] explains this twofold punishment as a reaction to two distinct categories of sin. The first involves harming the land itself, such as failing to observe the Sabbatical year and polluting the country with evil actions. The second category is a direct offense against the Temple.
The severity of their actions is highlighted by the distinction between defiling God's land and filling His inheritance with sin. The land refers to the entire Land of Israel, a place holier than all other lands and uniquely suited for God's presence [רד״ק]. The inheritance, however, points specifically to the absolute holiest sites where God dwells, most notably the Temple [מלבי״ם].
The people defiled these sacred spaces with idol worship, a practice understood as ugly, disgraceful, and repulsive, much like the carcasses of dead animals [רד״ק, מצודת ציון]. They did not merely limit their idolatry to the open countryside. Instead, they took their rebellion to the extreme by filling God's very inheritance with these repulsive objects. A prime example of this ultimate betrayal was King Manasseh, who went so far as to place a physical idol directly inside the sanctuary of the Temple itself [מלבי״ם].