But take utmost care and watch yourselves scrupulously

Whoever destroys one soul is considered by Scripture as if he had destroyed the entire world, and whoever sustains one soul is considered as if he had sustained the entire world

And this is not forbidden to the patient himself because it is a mitzvah, i.e., that the physician is obligated by law to heal the sick of Israel, and this is included in [the Sages’] commentary to the verse “and you shall restore it to him” (Deuteronomy 22:2) -- including his body, that if he saw that he is lost and could save him, then he must save him with his body or his money or his knowledge
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A baraita by the name of Rabbi Ishmael teaches that we are permitted to practice medicine… This “permission” is in fact a mitzvah, a commanded act, partaking of the duty to save life (pikuaḥ nefesh). One who is zealous in its performance is praiseworthy. One who delays or refuses is akin to a shedder of blood.

Every illness that the doctors say is dangerous, even though it is external on the flesh, they desecrate Sabbath for it. And if one doctor says [the treatment] is necessary [to save the patient's life] and one doctor says it is not necessary, they desecrate [the Sabbath by treating the patient].

If the patient does not want the medicine, we force the patient to take it.

When you build a new house, you shall make a parapet for your roof, so that you do not bring bloodguilt on your house if anyone should fall from it.

This requirement applies to a roof, and similarly, to any place that might present a danger and cause a person to stumble and die ... Similarly, it is a positive mitzvah to remove any obstacle that could pose a danger to life, and to be very careful regarding these matters, as Deuteronomy 4:9 states: "Beware for yourself; and guard your soul." If a person leaves a dangerous obstacle and does not remove it, he negates the observance of a positive commandment, and violates the negative commandment: "Do not cause blood to be spilled."

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.

5) (Vayikra 26:37) "And they will stumble, one man by his brother": It is not written "one man because of his brother" (i.e., in running), but "one man by his brother," the sin of his brother — whereby we are taught that all of Israel are responsible, one for the other.