I found this by dropping the search word “Kill” into http://bible.gospelcom.net/
16 The LORD said to Moses, 17 “Treat the Midianites as enemies and kill them, 18 because they treated you as enemies when they deceived you in the affair of Peor and their sister Cozbi, the daughter of a Midianite leader, the woman who was killed when the plague came as a result of Peor.”
1 The LORD said to Moses, 2 “Take vengeance on the Midianites for the Israelites. After that, you will be gathered to your people.” 3 So Moses said to the people, “Arm some of your men to go to war against the Midianites and to carry out the LORD’s vengeance on them. 4 Send into battle a thousand men from each of the tribes of Israel.” 5 So twelve thousand men armed for battle, a thousand from each tribe, were supplied from the clans of Israel. 6 Moses sent them into battle, a thousand from each tribe, along with Phinehas son of Eleazar, the priest, who took with him articles from the sanctuary and the trumpets for signaling.
7 They fought against Midian, as the LORD commanded Moses, and killed every man. … 9 The Israelites captured the Midianite women and children and took all the Midianite herds, flocks and goods as plunder. 10 They burned all the towns where the Midianites had settled, as well as all their camps. 11 They took all the plunder and spoils, including the people and animals, 12 and brought the captives, spoils and plunder to Moses and Eleazar the priest and the Israelite assembly at their camp on the plains of Moab, by the Jordan across from Jericho.
13 Moses, Eleazar the priest and all the leaders of the community went to meet them outside the camp. 14 Moses was angry with the officers of the army-the commanders of thousands and commanders of hundreds-who returned from the battle. 15 “Have you allowed all the women to live?” he asked them. 16 "They were the ones who followed Balaam’s advice and were the means of turning the Israelites away from the LORD in what happened at Peor, so that a plague struck the LORD’s people. 17 Now kill all the boys. And kill every woman who has slept with a man, 18 but save for yourselves every girl who has never slept with a man.