Kli Yakar interprets the sin as a deeper doubt in God's capability and the spies' mistake of judging based solely on human perspectives without faith in divine help.
אֶרֶץ אוֹכֶלֶת יוֹשְׁבֶיהָ הִוא. שֶׁרָאוּ אוֹתָם קוֹבְרִים מֵתִים, וְהָאֱלֹהִים חֲשָׁבָהּ לְטוֹבָה כוּ׳. וְקָשֶׁה, וְכִי בַּעֲבוּר זֶה נִקְרְאוּ מוֹצִיאֵי דִּבָּה, וַהֲלֹא הִגִּידוּ הָאֱמֶת מַה שֶּׁרָאוּ, וְכִי מִפְּנֵי שֶׁלֹּא הִרְגִּישׁוּ שֶׁלְּטוֹבָתָם עָשָׂה ה׳ זֹאת נִקְרְאוּ מוֹצִיאֵי דִבָּה, כְּסִילִים רָאוּי שֶׁיִּקָּרְאוּ. וְכֵן מַסִּיק בַּמִּדְרָשׁ (במ״ר טז ב): מְרַגְּלִים שֶׁשָּׁלַח מֹשֶׁה כְּסִילִים הָיוּ כוּ׳. וּלְפָחוֹת יֵשׁ מָקוֹם לוֹמַר אַהֲנֵי לֵיהּ שְׁטוּתֵיהּ שֶׁלֹּא חָלֵי וְלֹא מַרְגִּישׁ בְּטוֹבָתוֹ. וְנִרְאֶה שֶׁהָיָה לָהֶם לְכָל הַפָּחוֹת לִתְלוֹת הַקְּבוּרוֹת בְּחֶטְאָם, כִּי כְּבָר נוֹדַע לָהֶם שֶׁמַּעֲשֵׂה אֶרֶץ כְּנַעַן מְקֻלְקָלִים יוֹתֵר מִכָּל הָאֻמּוֹת, וְהֵם אָמְרוּ בְּהֵפֶךְ זֶה שֶׁכָּל הָעָם אֲשֶׁר רָאִינוּ בְּתוֹכָהּ אַנְשֵׁי מִדּוֹת טוֹבוֹת הֵמָּה וְצַדִּיקִים, וְתָלוּ הַדָּבָר בְּרוֹעַ מֶזֶג אֲוִיר הָאָרֶץ שֶׁאוֹכֵל וּמְכַלֶּה אֲפִלּוּ לְיוֹשְׁבֶיהָ הַמּוּרְגָּלִים בָּהּ, קַל וָחֹמֶר שֶׁתַּזִּיק לְבִלְתִּי יוֹשְׁבִים עָלֶיהָ וּבָאוּ מִמֶּרְחָק וְלֹא הוּרְגְּלוּ בַּאֲוִיר הָאָרֶץ. וּבְאׇמְרָם ״אַנְשֵׁי מִדּוֹת״ נִכְלְלוּ כַּמָּה דְּבָרִים, הֵן צִדְקָתָם וְיֹשֶׁר פְּעֻלָּתָם בְּמִדָּה וּבְמִשְׁקָל, אֵינָן יוֹצְאִין מִקַּו הַשִּׁוּוּי, הֵן עִנְיַן הַנְהָגָתָם בְּמַאֲכָלָם וּמִשְׁתָּם, שֶׁאֵינָן מִן הַמַּרְבִּים בְּמַאֲכָל וּמִשְׁתֶּה וּמִשְׁגָּל אֶלָּא מְכַלְכְּלִין דִּבְרֵיהֶם בְּמִשְׁפָּט בַּמִּדָּה וּבַזְּמַן הָרָאוּי, וְאִם כֵּן וַדַּאי הַכֹּל תָּלוּי בְּחוֹם הָאֲוִיר הַמְקַלְקֵל הַבְּרִיאוּת. ״וְזֶה פִּרְיָהּ״, כִּי בְּמָקוֹם שֶׁהַחוֹם גָּדוֹל אָז פִּרְיָהּ מְשֻׁנֶּה לְמַעְלְיוּתָא אֲבָל עַמָּהּ מְשֻׁנֶּה לִגְרִיעוּתָא, כִּי הָא בְּהָא תַּלְיָא. ״וְשָׁם רָאִינוּ אֶת הַנְּפִילִים״, מִן אוֹתָן שֶׁנָּפְלוּ וּמֵתוּ אֲשֶׁר קֻמְּטוּ בְּלֹא עֵת. ״בְּנֵי עֲנָק מִן הַנְּפִילִים״ הַקַּדְמוֹנִים, וְזֶהוּ כֶּפֶל ״הַנְּפִילִים״, כִּי אֶחָד תֹּאַר לַעֲנָקִים וְאֶחָד לְשׁוֹן נְפִילָה מַמָּשׁ. וְאָמְרוּ שֶׁאִם מֶזֶג הָאֲוִיר מַזִּיק לְגַסֵּי הַחוֹמֶר הַחֲזָקִים אַף בְּרִיאִים, קַל וָחֹמֶר שֶׁיַּזִּיק לָנוּ הַחֲלוּשִׁים וּקְטַנִּים מֵעֶרְכָּם. ״וַנְּהִי בְעֵינֵינוּ כַּחֲגָבִים וְכֵן הָיִינוּ בְּעֵינֵיהֶם״, כִּי שָׁמַעְנוּ אוֹמְרִים נְמָלִים יֵשׁ בַּכֶּרֶם, כִּי אֵינוֹ דּוֹמֶה רְאִיָּה מֵרָחוֹק לִרְאִיָּה מִקָּרוֹב, כִּי מֵרָחוֹק נִרְאֶה הַכֹּל יוֹתֵר קָטָן מִמָּה שֶׁהוּא נִרְאֶה מִקָּרוֹב, וַחֲגָבִים גְּדוֹלִים מִנְּמָלִים, וְאִם הָיִינוּ בְּעֵינֵינוּ כַּחֲגָבִים מִסְתָּמָא הָיִינוּ מֵרָחוֹק בְּעֵינֵיהֶם כִּנְמָלִים, כִּי אִידֵּי וְאִידֵּי חַד שִׁיעוּרָא הוּא. וְאָמְרוּ לָהֶם יְהוֹשֻׁעַ וְכָלֵב הָאָרֶץ טוֹבָה מְאֹד מְאֹד, הֵן מִצַּד פִּרְיָהּ, הֵן מִצַּד מֶזֶג הָאֲוִיר, וְאֵין טוֹבַת הַפֵּרוֹת תָּלוּי בְּחוֹם הָאֲוִיר כִּי אִם בְּשַׁמְנוּנִית הָאָרֶץ הַנּוֹתֵן מְתִיקוּת לַפֵּרוֹת. וְזֶהוּ שֶׁאָמְרוּ ״אֶרֶץ אֲשֶׁר הִיא זָבַת חָלָב וּדְבָשׁ״, כִּי הַבְּהֵמָה גִּדּוּלֵי קַרְקַע מִקְּרֵי, וְהַדְּבַשׁ כִּנּוּי אֶל הַמְּתִיקוּת הַבָּא מִשְּׁמַנֵּי הָאָרֶץ, וְהָאֲוִיר אֵינוֹ מַזִּיק כִּי אִם לַחוֹטְאִים. ״אַךְ בַּה׳ אַל תִּמְרֹדוּ״, וְאָז לֹא יַזִּיק לָכֶם אֲוִיר הָאָרֶץ. ״וְאַתֶּם אַל תִּירְאוּ אֶת עַם הָאָרֶץ״, שֶׁהָיוּ קוֹבְרֵי מֵתִים. ״כִּי לַחְמֵנוּ הֵם״, וַדַּאי בְּחֶטְאָם מֵתוּ, שֶׁהֲרֵי נֶאֱמַר (דברים ז טז): ״וְאָכַלְתָּ אֶת כָּל הָעַמִּים״, וְאָמְרוּ רז״ל (ב״ק לח.): מַאי דִּכְתִיב (חבקוק ג ו) ״רָאָה וַיַּתֵּר גּוֹיִם״, רָאָה הַקָּדוֹשׁ בָּרוּךְ הוּא שֶׁאֲפִלּוּ שֶׁבַע מִצְוֹת שֶׁל בְּנֵי נֹחַ לֹא קִיְּמוּ וְעָמַד וְהִתִּיר מָמוֹנָם לְיִשְׂרָאֵל. וְאִם מָמוֹנָם הִתִּיר עַל שֶׁלֹּא קִיְּמוּ שֶׁבַע מִצְוֹת אִם כֵּן גַּם גּוּפָם הִתִּיר עַל כָּל הַתּוֹעֵבוֹת הַגְּדוֹלִים שֶׁבָּהֶם, וְאִם כֵּן וַדַּאי בְּחֶטְאָם מֵתוּ, דְּאִם לֹא כֵן לָמָּה הִתִּיר לָנוּ לְאׇכְלָם כַּלֶּחֶם. וּרְאָיָה עַל חֶטְאָם שֶׁאֲפִלּוּ צַדִּיק אֶחָד שֶׁהָיָה בֵּינֵיהֶם מֵת בְּחֶטְאָם כְּדֵי שֶׁלֹּא יָגֵן עֲלֵיהֶם כְּצֵל. זֶהוּ שֶׁאָמְרוּ ״סָר צִלָּם מֵעֲלֵיהֶם״, אֲבָל צֵל שֶׁלָּנוּ קַיָּם, וְהוּא הַקָּדוֹשׁ בָּרוּךְ הוּא, שֶׁנֶּאֱמַר (תהלים קכא ה): ״ה׳ שֹׁמְרֶךָ ה׳ צִלְּךָ״.
It is a land that devours its inhabitants. They saw people burying the dead, and God meant it for good, etc. But this is difficult to understand: Why were they called spreaders of slander? After all, they merely reported the truth of what they saw. And just because they didn’t realize that God did this for their benefit, should they be called spreaders of slander? “Fools” would be a more appropriate term to call them, as the Midrash concludes (Bamidbar Rabbah 16:2): “The spies that Moses sent were fools, etc.” At the very least, one could say that their foolishness served them in that they neither sensed nor were aware of the benefit intended for them. It seems that they should have, at the very least, attributed the burials to their sin, since it was already known to them that the practices of the land of Canaan were more corrupt than all other nations. Yet they said the opposite — that all the people whom they saw there were people of good qualities and righteous, attributing the matter to the bad climate of the land which consumes and destroys even its inhabitants who are accustomed to it. How much more would it harm those who do not dwell in it but came from afar and are not accustomed to the air of the land. When they said people of good qualities [anshei midot, which is usually understood as people of people of great size], several things were included: their righteousness and the uprightness of their actions in measure [midot] and weight, not deviating from the line of exactitude; also the manner of their conduct in their food and drink, that they were not among those who indulge excessively in food, drink, and intimacy, but rather managed their affairs with judgment, in proper measure [midah] and at the appropriate time. If so, surely everything depended on the heat of the air which ruins health. And this is its fruit — because in a place where the heat is intense, the fruit is extraordinarily large, but along with this comes the extraordinary negative effect, for one depends on the other. And there we saw the Nephilim — referring to those who fell [naflu] and died, who were cut off before their time. The sons of Anak from the Nephilim — the ancient ones. This explains the repetition of Nephilim — one as a descriptor for the Anakim and one literally meaning “those who fell.” They said that if the climate harms even those of coarse constitution, the strong and healthy ones, how much more would it harm us who are weak and small in comparison to them. And we were in our own sight as grasshoppers, and so we were in their sight. For we heard them saying, “There are ants in the vineyard.” This is because seeing from a distance is not the same as seeing up close, as from a distance everything appears smaller than it does up close. Grasshoppers are larger than ants, and if we were like grasshoppers in our own sight, then presumably we were like ants in their sight from a distance, for both are of the same measure. And Joshua and Caleb said to them, “The land is very, very good.” Both in terms of its fruit and in terms of the climate. The quality of the fruits does not depend on the heat of the air but on the richness of the land that gives sweetness to the fruits. And this is what is meant by a land flowing with milk and honey — for livestock are considered products of the land, and honey is a reference to the sweetness that comes from the land’s richness. And the air is only harmful to sinners, but do not rebel against the Lord. And then the air of the land will not harm you, and you should not fear the people of the land. For they were burying their dead, because they are our bread. They certainly died because of their sins, as it is said, And you shall consume all the peoples (Deuteronomy 7:16). And our Sages said (Bava Kamma 38a): What is meant by the verse He looked and released the nations (Habakkuk 3:6)? The Holy One, blessed be He, saw that they did not even fulfill the seven Noahide laws, so He stood and permitted their property to Israel. And if He permitted their property because they did not fulfill the seven commandments, then He also permitted [taking] their bodies for all the great abominations they committed. So certainly they died because of their sin, for otherwise, why would He permit us to consume them like bread? And the proof of their sin is that even a single righteous person who was among them died because of their sin, so that he would not protect them like shade. This is what is meant by their shade has departed from them. But our shade remains, and that is the Holy One, blessed be He, as it is said, The Lord is your guardian; the Lord is your shade (Psalms 121:5).