מתני׳ קשרו בעליו במוסרה ונעל בפניו כראוי ויצא והזיק אחד תם ואחד מועד חייב דברי ר"מ רבי יהודה אומר תם חייב ומועד פטור שנאמר (שמות כא, לו) ולא ישמרנו בעליו ושמור הוא זה ר"א אומר אין לו שמירה אלא סכין:
גמ׳ [...] היינו טעמיה דר"א כדתניא ר' נתן אומר מניין שלא יגדל אדם כלב רע בתוך ביתו ואל יעמיד סולם רעוע בתוך ביתו שנאמר (דברים כב, ח) ולא תשים דמים בביתך:
MISHNA: If the ox’s owner tied it with reins to a fence or locked the gate before it in an appropriate manner, but nevertheless the ox emerged and caused damage, whether the ox is innocuous or forewarned the owner is liable, since this is not considered sufficient precaution to prevent damage; this is the statement of Rabbi Meir. Rabbi Yehuda says that if the ox is innocuous the owner is liable even if he safeguarded it appropriately, since the Torah does not limit the required safeguarding for an innocuous ox. But if the ox is forewarned, the owner is exempt from paying compensation for damage, as it is stated in the verse describing damage by a forewarned ox: “And the owner has not secured it” (Exodus 21:36), and this ox that was tied with reins or behind a locked gate was secured. Rabbi Eliezer says: An ox has no sufficient safeguarding at all other than slaughtering it with a knife; there is no degree of safeguarding that exempts the ox’s owner from liability.
GEMARA: [...] [T]his is the reason for the opinion of Rabbi Eliezer: As it is taught in a baraita that Rabbi Natan says: From where is it derived that one may not raise a vicious dog in his house, and that one may not set up an unstable ladder in his house? As it is stated: “You shall not bring blood into your house” (Deuteronomy 22:8), which means that one may not allow a hazardous situation to remain in his house. Similarly, a person should not keep a forewarned ox in his possession, as it is dangerous. This is why Rabbi Eliezer rules that no level of safeguarding is sufficient for it; the ox should be slaughtered so that it will not cause damage.
מתני׳ לא יצא האיש לא בסייף ולא בקשת ולא בתריס ולא באלה ולא ברומח ואם יצא חייב חטאת רבי אליעזר אומר תכשיטין הן לו וחכ"א אינן אלא לגנאי שנאמר (ישעיהו ב, ד) וכתתו חרבותם לאתים וחניתותיהם למזמרות ולא ישא גוי אל גוי חרב ולא ילמדו עוד מלחמה
MISHNA (6:4): The Sages said that a man may neither go out on Shabbat with a sword, nor with a bow, nor with a shield [teris], nor with an alla, nor with a spear. And if he accidentally went out with one of these weapons to the public domain he is liable to bring a sin-offering. Rabbi Eliezer says: These weapons are ornaments for him; just as a man is permitted to go out into the public domain with other ornaments, he is permitted to go out with weapons.
And the Rabbis say: They [weapons] cannot be seen as anything other than reprehensible and in the future they will be eliminated, as it is written: “And they shall beat their swords into plowshares and their spears into pruning hooks; nation will not raise sword against nation, neither will they learn war anymore” (Isaiah 2:4).
This next set of sources speak positively about the use of weapons for saving or protective lives.
If the thief is seized while tunneling, and he is beaten to death, there is no bloodguilt in his case.
גמ׳ אמר רבא מאי טעמא דמחתרת חזקה אין אדם מעמיד עצמו על ממונו והאי מימר אמר אי אזילנא קאי לאפאי ולא שביק לי ואי קאי לאפאי קטילנא ליה והתורה אמרה אם בא להורגך השכם להורגו
GEMARA: Rava says: What is the reason for this halakha [law] concerning a burglar who breaks into a house? He explains: There is a presumption that a person does not restrain himself when faced with losing his money, and therefore this burglar must have said to himself: If I go in and the owner sees me, he will rise against me and not allow me to steal from him, and if he rises against me, I will kill him. And the Torah stated a principle: If someone comes to kill you, rise and kill him first.
מתני׳ ואלו הן שמצילין אותן בנפשן הרודף אחר חבירו להרגו ואחר הזכר ואחר הנערה המאורסה [...]
גמ׳ ת"ר מניין לרודף אחר חבירו להרגו שניתן להצילו בנפשו ת"ל (ויקרא יט, טז) לא תעמוד על דם רעך והא להכי הוא דאתא האי מיבעי ליה לכדתניא מניין לרואה את חבירו שהוא טובע בנהר או חיה גוררתו או לסטין באין עליו שהוא חייב להצילו ת"ל לא תעמוד על דם רעך אין ה"נ.
MISHNA: And these are the ones who are saved from transgressing even at the cost of their lives; that is to say, these people may be killed so that they do not perform a transgression: One who pursues another to kill him, or pursues a male to sodomize him, or pursues a betrothed young woman to rape her. [...]
GEMARA: The Sages taught in a baraita: From where is it derived that with regard to one who pursues another in order to kill him, the pursued party may be saved at the cost of the pursuer’s life? The verse states: “You shall not stand idly by the blood of another” (Leviticus 19:16); rather, you must save him from death. The Gemara asks: But does this verse really come to teach us this? This verse is required for that which is taught in a baraita: From where is it derived that one who sees another drowning in a river, or being dragged away by a wild animal, or being attacked by bandits [listin], is obligated to save him? The Torah states: “You shall not stand idly by the blood of another.” The Gemara answers: Yes, it is indeed so that this verse relates to the obligation to save one whose life is in danger.