How is "Shomer Mitzvah Shomer Nafsho"?
R. Yonah: One who wants to guard his Nefesh, he will guard the Mitzvos, for they guard man's Nefesh - "Ki Hi Chayecha v'Orech Yamecha" (Devarim 30:20).
Malbim: There are two ways in front of man - of life and of death. He does not know in which he will go, for there is a straight path in front of a person (i.e. in his eyes), and its end is the way of death. He must heed the Commander, who commanded in which path to go and live. Then, he guards his Nefesh.
Why will "Bozeh Derachav" die?
Rashi: He does not put to his heart to weigh [his deeds].
R. Yonah: He scorns his ways and is not concerned; he does not put to his heart to fix them. There is more hope for a sinner who does not guard a Mitzvah, than for Bozeh Derachav. The former, his heart knows that he sins. If his Yetzer overpowers him today, perhaps Hashem will delay His anger and not rush to punish him for his sin. He will conquer his Yetzer and repent from his way. Bozeh Derachav is not concerned for fixing his Midos. He thinks that there is no harm in [bad] Midos, only in deeds. He will die without hope 1 , for he does not realize the evil of Midos. Someone with bad Midos cannot escape corrupt deeds. Also, bad Midos themselves are abhorred - "To'avas Hashem Kol Gevah Lev", "To'avas Hashem Sifsei Sheker" (16:5, 12:22).
Malbim: One who scorns his ways, and goes in the disgraceful ways of his heart and desire, he will die death of the Nefesh.
R. Yonah: Above, it taught that there is no hope for one who hates rebuke. He is worse than one who abandons the [good] path - "Musar Ra l'Ozev Orach v'Sonei Sochachas Yamus" (15:10).
