Babylon had just conquered Judah and Jerusalem, taken the king to Babylon as a captive, and put in a governor who would be accountable to Babylon. However, some thought to take advantage of the situation, and they killed the new governor and anyone who came to support him,

And it happened, on the second day after he had killed Gedaliah, when as yet no one knew it, that certain men came from Shechem, from Shiloh, and from Samaria, eighty men with their beards shaved and their clothes torn, having cut themselves, with offerings and incense in their hand, to bring them to the house of the Lord. Now Ishmael the son of Nethaniah went out from Mizpah to meet them, weeping as he went along; and it happened as he met them that he said to them, “Come to Gedaliah the son of Ahikam!” So it was, when they came into the midst of the city, that Ishmael the son of Nethaniah killed them and cast them into the midst of a pit, he and the men who were with him (Jeremiah 41.4–7).

These evil men believed that with Judah’s government fallen, and the Babylonian government hundreds of miles away, they could do as they pleased, but the rest of the Book of Jeremiah shows that the justice of God found those assassins and punished them.