Tafsiran Keluaran 22:1: Rashi, Chizkuni, Da'at Zekenim & Rashbam

אם במחתרת means when he was breaking in into the house (i. e. in the very act of forcing an entry, but at no other moment).

אם במחתרת ימצא הגנב, “if the thief was discovered while tunneling” (acting so as not to be discovered); according to Rashi he had left behind evidence of forcible entry into the victim’s house;

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.

אין לו דמים THERE SHALL NO GUILT OF BLOOD BE INCURRED FOR HIM — This is not regarded as a murder; it is as though he (the thief) has been dead from the beginning of his criminal act (אין לו דמים is taken to mean: he, the thief, had no blood — no vitality). Here the Torah teaches you the rule: “If one comes with the intention of killing you, be quick and kill him”. — And this burglar actually came with the intention of killing you, for he knew full well that no one can hold himself in check, looking on whilst people are stealing his property before his eyes and doing nothing. He (the thief) therefore obviously came with this purpose in view — that in case the owner of the property would resist him, he would kill him (Sanhedrin 72a).