Our Gemara on Amud Aleph tells us an interesting story about King Yannai, a Hasmonean Jewish king:
An incident occurred with King Yannai, who went to the region of Koḥalit in the desert and conquered sixty cities there. And upon his return, he rejoiced with great happiness over his victory. He subsequently summoned all the Sages of the Jewish people and said to them: Our ancestors, in their poverty, would eat salty foods when they were busy with the building of the Temple; we too shall eat salty foods in memory of our ancestors. And they brought salty food on tables of gold and ate.
At the goading of a certain scoffer who knew that this would provoke enmity, King Yannai also decided to wear the Tzitz, the Golden headband of the high priest. Now there was a certain elder present called Yehuda ben Gedidya, and Yehuda ben Gedidya said to King Yannai: King Yannai, the crown of the monarchy suffices for you, i.e., you should be satisfied that you are king. Leave the crown of the priesthood for the descendants of Aaron. The Gemara explains this last comment: As they would say that Yannai's mother was taken captive in Modi'in, and she was therefore disqualified from marrying into the priesthood, which meant that Yannai was a ḥalal.
King Yannai, hesitant to annihilate the sages, since then, who would be left to preserve the Torah? The scoffer also succeeded in convincing King Yannai about this as well, by arguing that the written Torah was available for anyone to read. What about the Oral Torah, wonders Yannai momentarily? The Gemara says, "נִזְרְקָה בּוֹ מִינוּת" "Thoughts of Heresy were thrown into him," meaning that he doubted the existence of the Oral Torah, allowing himself to execute the sages without qualms.
Yannai seems to start out with good intentions, showing gratitude and commemorating the poverty of his ancestors. How did everything end up so badly? And, how did he go so quickly from humility, to the arrogance of wearing the Tzitz, to heresy? We must conclude that his original modesty was false. He deluded himself into thinking he was humble when his sincerity was only one centimeter deep. And when slighted, he quickly rationalized murder, abrogated rabbinic authority and the oral tradition.
The idiom "נִזְרְקָה בּוֹ מִינוּת" "Thoughts of Heresy were thrown into him" is evocative and linguistically pleasing. It seems to be saying that heretical thoughts are not core to the human mind; rather, it is an external demon of sorts that is thrown into him, and possesses him. Siach Sarfei Kodesh (2:232) also reacts to this idiom; however, he says that it refers to the slippery slope of heresy. First, it is a small drop that enters, and it is easy to let it slide, but it can eventually erupt into full-blown blasphemy. His interpretation of this phrase fits well into the way we described King Yannai's spiritual downfall. At first, some insincerity and arrogance, but eventually full-blown self-worship.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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