They use the story of Amalek from the Torah.
One of the Rabbis I watched recently said "when you kill the first child it breaks your heart [...] then you start to enjoy it."
_Many_ Rabbis are demanding that animals, children, women and unarmed males be "erased." IDF soldiers are bragging about killing and raping civilians on social media. One IDF soldier was complaining he hasn't shot any children under 12 yet.
Netanyahu is a moderate. He's not an "extremist."