Gen

Gen 34:13-17

Jacob's sons answered Shechem and his father with cunning. Their sister, after all, had been raped. 14 They said, "This is impossible. We could never give our sister to a man who was uncircumcised. Why, we'd be disgraced. 15 The only condition on which we can talk business is if all your men become circumcised like us. 16 Then we will freely exchange daughters in marriage and make ourselves at home among you and become one big, happy family. 17 But if this is not an acceptable condition, we will take our sister and leave."  

Since something VERY DEAR AND PERSONAL TO THEM had been violated they chose to attack them back with something that was equally as dear and personal!