His Students Rebuked Him for His Bad Middos:

Rashi states: "This teaches us that he was haughty." This seems to indicate that if he hadn't said these words, we wouldn't have known that Bilaam was haughty. This means that he hid his haughtiness and pretended to be a righteous person. From looking at him, no one could tell that he was a baal gaivoh, but one could discern this from observing his students. From seeing how they acted, one could tell that their teacher must be a bad person. Rav Yechezkel of Kuzmir zy "a (quoted in Divrei Yisroel, Klalei Ohraysa, Ois 40) discerns this from the Mishnah (Avos 5:19) that says: "What is the difference between the students of Avrohom Avinu and the students of Bilaam the wicked..." Why does it only ask this question about the students? Why doesn't it list the difference between Bilaam and Avrohom themselves? The answer is that based on outsider appearances, there was no difference between them. Bilaam was also a Rebbe. To outsiders he looked like a pious person. However, from seeing how his students acted, one could tell that there was a huge difference between him and Avrohom Avinu.