The moral decay of the people reaches a horrifying peak, blending gross idolatry with a blatant assault on the holiness of God. The tragedy lies in the shocking overlap of their actions. On the exact same day that they slaughtered and sacrificed their children to idols, they proceeded to the Temple to offer sacrifices to God.
This act of defiling the sanctuary is understood in a few ways. One approach highlights their sheer defiance and hypocrisy. They entered the Temple specifically to commit an abomination and provoke the anger of God [רש״י]. They behaved as though God was blind to the horrific deeds they had just committed, which inherently turned their Temple offerings into an impurity [רד״ק]. Sacrifices brought by individuals guilty of such atrocities are entirely unwanted by Him [מצודת דוד]. Alternatively, their arrival at the Temple immediately after murdering their children was not merely an act of moral blindness or hypocrisy. Rather, it was a calculated action designed explicitly to contaminate the holy site [מלבי״ם].
The tragedy is further compounded by the timing of these events. The primary approach among commentators is that the atrocity of slaughtering their children for idols was carried out on the Sabbath. Through these actions, the people orchestrated a deliberate attack on three distinct dimensions of holiness. They corrupted the dimension of space by contaminating the Temple, they violated the dimension of time by profaning the Sabbath, and they destroyed the dimension of the soul by murdering a child who was meant to be dedicated to God and the study of Torah [מלבי״ם].