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אם במחתרת. משמע דוקא כשבא דרך מחתרת שחתר בבית ליכנס אבל נכנס דרך פתח דמים לו דודאי אינו בא להרוג שאומר בלבו אם יבא בעל הבית לעמוד לפני ממונו שיברח הוא דרך הפתח: אם במחתרת ימצא הגנב, “if the thief is discovered while tunneling, underground, etc.;” the scenario described here where the owner of the house being burgled kills the intruder, and the Torah exonerates him, applies only when the thief had made an effort to remain undiscovered. If he broke in the front door and the owner harms or kills him, he is liable for damages or even murder, seeing that his life had not been endangered. The thief had obviously decided that if discovered he would flee the way he had entered, and not become violent.