Adam Clarke on Jeremiah 29:21

He shall slay them before your eyes - Nebuchadnezzar would be led by political reasons to punish these pretended prophets, as their predictions tended to make his Israelitish subjects uneasy and disaffected, and might excite them to rebellion. He therefore slew them; two of them, it appears, he burnt alive, viz., Ahab and Zedekiah, who are supposed by the rabbins to be the two elders who endeavored to seduce Susanna, see Jer 29:23. Burning alive was a Chaldean punishment, Dan 3:6, and Amo 2:1. From them other nations borrowed it.