Jehoiachin was eighteen years old when he became king. But he ruled for only three months and ten days in Jerusalem. In God's opinion he was an evil king.
In the spring King Nebuchadnezzar ordered him brought to Babylon along with the valuables remaining in The Temple of God. Then he made his uncle Zedekiah a puppet king over Judah and Jerusalem.
Zedekiah was twenty-one years old when he started out as king. He was king in Jerusalem for eleven years.
As far as God was concerned, he was just one more evil king; there wasn't a trace of contrition in him when the prophet Jeremiah preached God's word to him.
Then he compounded his troubles by rebelling against King Nebuchadnezzar, who earlier had made him swear in God's name that he would be loyal. He became set in his own stubborn ways - he never gave God a thought; repentance never entered his mind.
The evil mindset spread to the leaders and priests and filtered down to the people - it kicked off an epidemic of evil, repeating the abominations of the pagans and polluting The Temple of God so recently consecrated in Jerusalem.