How is killing of civilians and children – even infants – justified? 

Rabbis Yitzhak Shapira and Yosef Elitzur will not be prosecuted for their book Torat HaMelech. Shapira and Elitzur wrote in Torat HaMelech that non-Jews who are non-combatants could be killed in times of war. Even babies in their cribs could be killed, they wrote,  “if there is a good chance they will grow up to be like their evil parents,” the Jerusalem Post reported.

What would you do if it was your children?

More Than 1,500 Children Killed by Israeli Forces – International Middle East Media Center.