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1. Nothing happens against Hashem's will.


2. Ergo, when one sins, that's also Hashem's will.


3. Yet we know that Hashem wants us not to sin, so it's against His will.

Contradiction?

From Rabbi Uri Sherqi:

1. In the words of Rabbi Tzvi Yehudah Kook, “Hashem needs their to be evil in the world. But, there’s no mizwah to volunteer.”


2. If someone were to commit a sin absolutely Leshem Shamayim, it may be considered a mizwah. The problem is that there is a hazaqah on mankind that sins Leshem Shamayim are incredibly rare. And, if you commit the sin not Leshem Shamayim, you’ll pay the price for it.

In short, we are required to abide by what Hashem told us is mutar and assur.

Rambam Hilchot Teshuvah 5:4

ואל תתמה ותאמר היאך יהיה האדם עושה כל מה שיחפוץ ויהיו מעשיו מסורים לו וכי יעשה בעולם דבר שלא ברשות קונו ולא חפצו והכתוב אומר כל אשר חפץ ה' עשה בשמים ובארץ דע שהכל כחפצו יעשה ואף על פי שמעשינו מסורין לנו כיצד כשם שהיוצר חפץ להיות האש והרוח עולים למעלה והמים והארץ יורדים למטה והגלגל סובב בעיגול וכן שאר בריות העולם להיות כמנהגן שחפץ בו ככה חפץ להיות האדם רשותו בידו וכל מעשיו מסורין לו ולא יהיה לו לא כופה ולא מושך אלא הוא מעצמו ובדעתו שנתן לו האל עושה כל שהאדם יכול לעשות

A person should not wonder: How is it possible for one to do whatever he wants and be responsible for his own deeds? - Is it possible for anything to happen in this world without the permission and desire of its Creator as [Psalms 135:6] states: "Whatever God wishes, He has done in the heavens and in the earth?"

One must know that everything is done in accord with His will and, nevertheless, we are responsible for our deeds.

How is this [apparent contradiction] resolved? Just as the Creator desired that [the elements of] fire and wind rise upward and [those of] water and earth descend downward, that the heavenly spheres revolve in a circular orbit, and all the other creations of the world follow the nature which He desired for them, so too, He desired that man have free choice and be responsible for his deeds, without being pulled or forced. Rather, he, on his own initiative, with the knowledge which God has granted him, will do anything that man is able to do.

(Touger translation, my emphasis)

Essentially, then, God's will is that humans have the free choice to act against His will.

Assumptions 1, and therefore 2, are at least partly false. That nothing happens against the divine will is only true when one looks at the contextual greater picture. But without context i.e. at the "micro" level, choices are made against His will all the time, whenever people sin. He allows this for the same reason that He allows suffering - as part of a larger purpose, primarily free will.

Saying that sin is according to His will is the same as saying that a parent who wants a child to learn to be independent, also wants the failures and suffering that inevitably result.