Self-nullification gives Torah eternal life

איתא אין התורה מתקיימת אלא במי שממית עצמו עלי'.

It is taught: the Torah endures only in one who "kills himself" over it (Berachos 63b).

True acquisition of Torah comes through total self-nullification (bittul) — a person must give himself over to it completely, as though laying down his very life.

הגם דכתיב וחי בהם ולא שימות בהם.

Even though it is written, "and he shall live by them" (Vayikra 18:5) — and Chazal expound: "by them," and not that he should die through them.

This seems to contradict the first statement: the mitzvos are meant to give life, not to be a cause of death.

אך אדרבה זה עצמו הפי' אשר יעשה אותם כו' וחי בהם בלבד לא בדברים אחרים כאשר קיימו אבותינו בהר סיני שיצאה נשמתן וניתן להם חיות חדש ונעשו כברי' חדשה ותרי"ג מצות נתנו להם חיות לרמ"ח איברים ושס"ה גידין ולכן נתקיים התורה בבנ"י כיון שהמיתו עצמן עלי':

But on the contrary, this itself is the meaning: "which a person shall do… and live by them" — by them alone and not in other things; just as our forefathers did at Har Sinai, where their souls departed and they were given a new vitality and became as new creations, and the taryag (613) mitzvos gave life to their 248 limbs and 365 sinews. Therefore the Torah took root in Bnei Yisrael, since they "killed themselves" over it.

The two ideas are really one. "Living by them" means drawing all of one's life from the Torah alone. At Har Sinai the people died to their old selves — their neshamos departed — and Hashem gave them a new chiyus (life-force), so that the 613 mitzvos animate the 613 parts of the body. The very self-surrender, the "death" over Torah, is what allowed the Torah to live permanently within Bnei Yisrael.

Summary: "Killing oneself" over Torah and "living by the mitzvos" are not contradictions but the same truth — by nullifying his old self entirely to the Torah, as Bnei Yisrael did at Sinai, a person receives a new, higher chiyus, and the Torah becomes the source of life for his every limb.