The Mishnah and Gemara in the 8th perek of Sanhedrin discuss the case of הבא במחתרת - a burglar who tunnels or otherwise breaks into one's home. The Torah in Parshas Mishpatim says that the homeowner is not liable if he kills such an intruder, and the Mishnah and Gemara elaborate on this case.

In the Gemara in Sanhedrin דף עב ע"א, רבא states that one may kill the intruder because  "חזקה אין אדם מעמיד עצמו על ממונו"- "a person will not stand by while his property is taken". Rashi and the Yad Ramah approach the sugya somewhat differently, but they both explain that the intruder is surely intent on killing the homeowner in the event that the two confront each other, as חזקה אין אדם מעמיד עצמו על ממונו means that the homeowner will resist, and the intruder thus breaks in planning to kill if or when he meets the homeowner during the break-in. Rashi in יומא דף פה ע"ב emphasizes that this is not a case of ספק נפשות - of a mere doubtful case of self-defense - but that it is ודאי נפשות - a case of definite self-defense; the intruder is treated as a definite רודף - one who pursues another and threatens his life - whom the Torah rules may be killed.

However, the Rambam presents the דין of הבא בחמתרת differently. In the 9th פרק of הלכות גניבה, the Rambam writes: "ומפני מה התירה תורה דמו של גנב אף על פי שבא על עסקי ממון? לפי שחזקה שאם עומד בעל הבית ומנעו יהרגנו, ונמצא זה הנכנס לבית חבירו כרודף"- "Why did the Torah permit one to kill the intruder even though he only came to steal money? Because it is presumed that if the homeowner confronts the intruder and tries to stop him, the intruder will kill the homeowner; the intruder is thus like a lethal pursuer who may be killed."

It must be noted that the Rambam writes, "לפי שחזקתו שאם עומד בעל הבית ומנעו יהרגנו"- "it is presumed that if the homeowner confronts the intruder and tries to stop him, the intruder will kill the homeowner". The Rambam differs from Rashi and the Ramah, both of whom emphasize that this is not a case of ספק נפשות and explain that חזקה אין אדם מעמיד עצמו על ממונו refers to the surety that the homeowner will resist and that the intruder thus enters ready to kill. The Rambam, on the contrary, learns that the chazokoh refers to a presumption about the intruder rather than the homeowner - that it is presumed that the intruder will kill if the homeowner resists - thereby leaving open the possibility that should the intruder meet the homeowner and the homeowner would ignore the intruder or cooperate with his demands, the intruder will not kill.

The Rambam evidently does not consider this to be a case of vadai nefoshos; how then may the homeowner take the intruder's life?

The answer seems to be that the Rambam has a different conception of הבא במחתרת. The Rambam holds that the intruder is not an actual רודף, but rather that he has a din of a רודף - that the Torah conferred upon the intruder the status of a full-blown רודף even though he practically may not qualify as a רודף.

This is why the Rambam writes that"ונמצא זה הנכנס לבית חבירו כרודף"  - "that an intruder is like a Rodef - for the Rambam maintains that the intruder is not an actual רודף, but rather that the Torah bestows upon him the legal status or דין of a רודף. In contrast, it is clear from Rashi and the Ramah that they consider the intruder to be an actual רודף; Rashi says it straight out in Sanhedrin דף עב ע"ב, writing that "רודף הוא" - the intruder is an actual רודף.

One important נפקא מינה, or difference, between the approaches of Rashi and the Yad Ramah versus the Rambam relates to התראה - warning the intruder before taking action against him. Although the Gemara says that "מחתרתו זו היא התראתו"- "His Tunneling itself serves as his warning", Rashi and the Ramah explain that although an intruder who tunnels in does not need התראה before the homeowner may strike him, an intruder who enters through a patio, courtyard or other opening does indeed require התראה, as it is not clear that he enters with the intent to kill. However, as the Maggid Mishneh demonstrates, the Rambam does not distinguish between intruders, holding that all intruders, regardless of whether they tunnel in or enter through normal entrances, never need התראה.

This מחלוקת, or dispute, about התראה, between Rashi and the Ramah versus the Rambam is לשיטתם - it fits into their dispute about the halachic character of an intruder. Whereas Rashi and the Ramah hold that an intruder would be a regular רודף and therefore theoretically needs התראה (and that only one who intrudes by tunneling in needs no התראה), the Rambam maintains that an intruder is different than a regular רודף, for an intruder receives an automatic דין of a full-blown רודף as conferred upon him by the Torah, and he can thus be acted against without התראה.

According to Rashi and the Ramah, who opine that an intruder is a regular רודף, the homeowner would have to try to strike the intruder in a way that is not lethal, if possible, like any רודף. However, the Rambam omits this requirement. It would appear that since the Rambam learns that an intruder is not an actual רודף, but that the Torah bestows upon him a new din, that he is כרודף, he may be killed forthright.

It is also noteworthy that whereas the Rambam discusses the various types of רופים and the details of their דינים in the first perek of הלכות רוצח, he omits הבא במחתרת there and discusses it instead in הלכות גניבה. This further demonstrates that the Rambam classifies הבא במחתרת as a new halachic category rather than as a regular רודף, for if הבא במחתרת were a regular רודף, the Rambam should have included it with the other types of רודפים discussed in הלכות רוצח.

Our approach in the Rambam also answers a question posed by Tosafos on דף עג ע"א in סנהדרין. Tosafos asks why the rule that one who pursues another to kill him may be killed requires a separate דרשה, an independent source in the Torah, and is not learned from the פסוק, the verse in שמות, which states that an intruder may be killed. Tosafos asked this question because he held that an intruder is a regular רודף; however, the Rambam would not be bothered by Tosafos' question, as the Rambam maintains that הבא במחתרת is not a regular רודף, and hence the rules for a regular רודף could not be derived from his case.

One final point about רודף: The Ramah and Rashi indicate that one who is clearly a רודף needs no התראה. On the other hand, the Rambam in הלכות רוצח פרק א' הלכה ז' does require התראה for a regular, unmistakable רודף, albeit that the רודף need not accept the התראה. Now, since the התראה need not be accepted by the רודף, and even a child who is a רודף can be killed, it cannot be said that the Rambam views killing the רודף as a punishment and that the Rambam thus requires התראה as a prerequisite for such. Rather, we may explain that the Rambam holds that a רודף  is not a mere description of one who pursues to kill; to be precise, we can suggest that the Rambam maintains that רודף  is a דין in the גברא, the person, and that התראה which is given to one who pursues to kill converts the person into a גברא of a רודף  - unlike how other Rishonim learn, that רודף  is a mere description of one who pursues to kill and is not a new halachic personality. (The Ramah in Sanhedrin דף עג ע"א is clear that רודף is a mere descriptive term and is not a new type of גברא.)