1)
Why did he kill all of them?
1.
Malbim: It was so nothing would remain from Beis Yehoram [ben Yehoshafat], the son-in-law of Achav.
2)
Why did he say to seize them alive, if he would kill all of them?
1.
Radak: This was a greater vengeance, not to kill them in the way of war.
