What did the king of Israel want to do with the captured Aramean army?

Answer: Kill them

Seeing the Arameans captive in Samaria, the king of Israel asked Elisha, 'My father, shall I kill them? Shall I kill them?'

“And when the king of Israel saw them, he asked Elisha, “My father, shall I kill them? Shall I kill them?””— 2 Kings 6:21 (BSB)