Musar for Sanhedrin 143:7

ולא היא כי אוקמינא רחמנא ברשותיה לענין אונסין אבל לענין מקנא ברשותיה דמרייהו קיימי מידי דהוה אשואל

<b><i>GEMARA</i></b>. Raba said: what is the reason for the law of breaking in? Because it is certain that no man is inactive where his property is concerned; therefore this one [the thief] must have reasoned, 'If I go there, he [the owner] will oppose me and prevent me; but if he does I will kill him.' Hence the Torah decreed, 'If he come to slay thee, forestall by slaying him'.

Mesilat Yesharim