They offered up their children to demonic “gods,” “Molech” being the main one, and murdered them.

The people of Judah have done evil in my eyes, declares the LORD. They have set up their detestable idols in the house that bears my Name and have defiled it.

They have built the high places of Topheth in the Valley of Ben Hinnom to burn their sons and daughters in the fire—something I did not command, nor did it enter my mind. (Jeremiah 7:30-31)

They have built the high places of Baal to burn their sons in the fire as offerings to Baal—something I did not command or mention, nor did it enter my mind. (Jeremiah 19:5)

They built high places for Baal in the Valley of Ben Hinnom to sacrifice their sons and daughters to Molech, though I never commanded, nor did it enter my mind, that they should do such a detestable thing and so make Judah sin. (Jeremiah 32:35)

https://healthimpactnews.com/2021/what-happens-when-a-holy-and-righteous-god-gets-angry-lessons-from-history-and-the-prophet-jeremiah/