Not everybody in Judah was evil.
Certainly Jeremiah was not.
And the destruction of the temple and deportation of nearly everyone was not all bad. In fact, either group (those taken or those left behind) could have seen it as an opportunity. After all, had not Isaiah talked about a faithful remnant? Had not Ezekiel and Jeremiah? Either group could have claimed the title and acted in faithfulness – but neither did.
When the Babylonians left the smoking hulk that was Judah, they knew they’d not captured everyone. Some would be left behind, hiding in caves, self-exiled to the surrounding nations. They left Gedaliah, a good man, son of Ahikam (protector of Jeremiah – 26:24) and descendant of Shaphan (secretary to Josiah), to watch over and administer the province. People came out of the woodwork and Gedaliah told them to settle down, be obedient to the Babylonians and work together. Crops remained in the fields and if they were to eat, the crops must be harvested. Gedaliah brought them all under the protection of the king of Babylon. All they had to do was be obedient and work. Things would go well for them.
Perhaps because of the trials of the time, Gedaliah wanted to believe the best about everyone and ignored the rumblings that Ishmael was plotting to kill him. It was, of course, a mistake, but it reminds us of just how evil the people of Judah had become. Not everyone, of course, but enough of them that the tide of civility, and all hope of being the remnant was lost.