Rashi on Exodus 22:1/רש"י על שמות כ״ב:א׳

1אם 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).

2אין 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).