Our Gemara on Amud Aleph discusses the intentions of the Spies:

וַיֵּלְכוּ וַיָּבֹאוּ אָמַר רַבִּי יוֹחָנָן מִשּׁוּם רַבִּי שִׁמְעוֹן בֶּן יוֹחַי מַקֵּישׁ הֲלִיכָה לְבִיאָה מָה בִּיאָה בְּעֵצָה רָעָה אַף הֲלִיכָה בְּעֵצָה רָעָה

And they went and they came” (Numbers 13:25–26). Rabbi Yoḥanan says in the name of Rabbi Shimon ben Yoḥai: This verse likens their going to their coming. Just as their coming back was with wicked counsel, so too, their going to Eretz Yisrael was with wicked counsel.

What exactly was this “wicked counsel?”  It must have been unusually compelling because our Gemara says (34b) that Moshe had to pray to save Yehoshua from the influence of the Spies. I will offer a peshat, based on how the Gemara describes the ultimate fate of the Spies:

וַיָּמֻתוּ הָאֲנָשִׁים מוֹצִאֵי דִבַּת הָאָרֶץ רָעָה בַּמַּגֵּפָה אָמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ שֶׁמֵּתוּ מִיתָה מְשׁוּנָּה אָמַר רַבִּי חֲנִינָא בַּר פָּפָּא דָּרֵשׁ רַבִּי שֵׁילָא אִישׁ כְּפַר תְּמַרְתָּא מְלַמֵּד שֶׁנִּשְׁתַּרְבֵּב לְשׁוֹנָם וְנָפַל עַל טִיבּוּרָם וְהָיוּ תּוֹלָעִים יוֹצְאוֹת מִלְּשׁוֹנָם וְנִכְנָסוֹת בְּטִיבּוּרָם וּמִטִּיבּוּרָם וְנִכְנָסוֹת בִּלְשׁוֹנָם

The verse states: “And those men who brought out an evil report of the land, died by the plague before the Lord” (Numbers 14:37). Rabbi Shimon ben Lakish says: This means that they died an unusual death. Rabbi Ḥanina bar Pappa says that Rabbi Sheila Ish Kefar Temarta taught: This teaches that their tongues were stretched out from their mouths and fell upon their navels, and worms were crawling out of their tongues and entering their navels, and worms were likewise coming out of their navels and entering their tongues. This is the painful death that they suffered. 

Kli Yakkar Bamidbar (14:37) notes the circular quality to this Aggadah.  In utero, the fetus is nourished via the navel, and then the mouth takes over this function after birth. This hints at the Spies’ improper intentions from the very beginning. I would add, the navel is the beginning of the formation of the child, also hinting at beginnings and origins,

Perhaps the Aggadah is teaching that a negative disposition at the beginning becomes a self-fulfilling prophecy. The circular process of the tongue to the navel and the navel to the tongue hints at the Spies’ improper attitude, which influenced their speech, their continued attitude and the final outcome. Jewish theology and common sense tell us that what we think and what we say affects outcomes, and I believe is reflected in the following Talmudic dictum (Kesuvos 8b):

אָמַר רַבִּי שִׁמְעוֹן בֶּן לָקִישׁ, וְכֵן תָּנָא מִשְּׁמֵיהּ דְּרַבִּי יוֹסֵי: לְעוֹלָם אַל יִפְתַּח אָדָם פִּיו לַשָּׂטָן. אָמַר רַב יוֹסֵף, מַאי קְרָא: ״כִּסְדוֹם הָיִינוּ לַעֲמוֹרָה דָּמִינוּ״. מַאי אַהְדַּר לֵיהּ — ״שִׁמְעוּ דְבַר ה׳ קְצִינֵי סְדוֹם וְגוֹ׳״.

Shimon ben Lakish said, and likewise it was taught in the name of Rabbi Yosei: A person should never open his mouth to Satan and speculate about potential disasters. Rav Yosef said: What is the verse from which it is derived? “We should have almost been as Sodom, we should have been like unto Gomorrah” (Isaiah 1:9), after which, what did the prophet reply to them? “Hear the word of the Lord, rulers of Sodom; give ear unto the law of our God, people of Gomorrah” (Isaiah 1:10). Isaiah drew the analogy and immediately it was realized.

What does the research say about Self-Fulfilling Prophecies?  Especially regarding interpersonal interactions, attitudes are highly influential toward outcomes. Researchers Stukas and Snyder (“Self-Fulfilling Prophecies”, (2016) In H. S. Friedman (Ed), Encyclopedia of mental health (2nd edition, Vol. 4, pp. 92-100). San Diego, CA: Academic Press.) report:

People typically enter their social interactions with preconceived beliefs and expectations about how other people will act and they often use these beliefs as guides for their own actions with these others. These actions, in turn, may prompt their interaction partners to behave in ways that confirm the initial beliefs.

…expectations may anticipate behaviors based on each other's appearances, personalities, attitudes, preferences, skills and abilities, goals, mood states, or a host of other salient features. Although beliefs about how a person will act can come from past experiences with the person, they may also arise from stereotypes related to the social categories to which a person belongs, such as those related to age, ethnicity, gender, and occupation, among many other categories. Once activated, these expectations may influence how people choose to act with each other. Such behavioral choices may consequently influence the other person's behavior in turn. For example, if we hold an expectation that a person is smart, even erroneously, we may choose conversational topics that allow them to talk about intellectual things, giving them a chance to demonstrate intelligence. Whereas, if we think a person is less smart, we might guide the conversation toward less sophisticated topics, thereby gathering only evidence that the person can talk about undemanding things. When beliefs influence, or dictate, how another person acts such that the person comes to confirm the initial beliefs of their interaction partner with their behavior, we say that a self-fulfilling prophecy has occurred

For example, Rosenthal and Jacobsen (1968, “Pygmalion in the Classroom”, Teacher expectation and pupils’ intellectual development. New York, NY: Holt, Rinehart and Winston) provided teachers with information that suggested that a randomly assigned subset of students in their class were "late bloomers" and would increase in academic ability over the course of the coming school year. As compared to students for whom no new expectations were provided, these students for whom teachers held positive expectations actually did demonstrate improvements in performance. This outcome is often termed the "Pygmalion" effect (after the George Bernard Shaw play) because of its revelation of the potential ability of perceivers' treatment to transform targets in a positive way. Although the gains by students in the original study were not sustained in subsequent years, Rosenthal and his colleagues have used additional experimental studies to more clearly isolate those behaviors of teachers most responsible for the effects

…Idealized perceptions of targets can have beneficial effects. For example, within close relationships, overly positive perceptions of a romantic partner are associated with greater relationship satisfaction for both partners (e.g., Murray, Holmes, & Griffin, 1996). Indeed, through what has been called the Michelangelo effect (e.g., Drigotas, Rusbult, Wieselquist, & Whitton, 1999), perceivers may enable their romantic partners to come closer to their ideal selves by treating their partners in line with such overly positive expectations. As we have seen, in the workplace as well, supervisors' positive expectations may influence employees' perceptions of what they are capable (see Eden, 2003).