After killing Zedekiah's sons before him, what did the Babylonians do to him?

Answer: Put out his eyes

At Riblah the Babylonians slaughtered Zedekiah's sons before his eyes, then put out his eyes, bound him in bronze shackles and took him to Babylon.

“The Chaldeans seized the king and brought him up to the king of Babylon at Riblah, where they pronounced judgment on him. And they slaughtered the sons of Zedekiah before his eyes. Then they put out his eyes, bound him with bronze shackles, and took him to Babylon.”— 2 Kings 25:6-7 (BSB)