How did Hazael kill Ben-hadad king of Aram?

Answer: Smothered him with a wet cloth

The day after visiting Elisha, Hazael dipped a thick cloth in water and spread it over the king's face; Ben-hadad died and Hazael reigned.

“So Hazael left Elisha and went to his master, who asked him, “What did Elisha say to you?” And he replied, “He told me that you would surely recover.” But the next day Hazael took a thick cloth, dipped it in water, and spread it over the king’s face. So Ben-hadad died, and Hazael reigned in his place.”— 2 Kings 8:14-15 (BSB)