It is the general consensus of the academic community that Rambam denied the existence of demons. Amongst traditional scholars, those who accepted that Rambam denied the existence of demons include the Gerona kabbalist R. Shlomo b. Meshullam da Piera, R. Yosef b. Shem Tov, R. Yosef Shlomo Delmedigo, R. Aviad Sar-Shalom Basilea, Abarbanel, the Vilna Gaon, R. Yosef Ergas, R. Yosef Shaul Nathanson, R. Menashe ben Yisrael, R. Eliezer Neusatz, R. Eliezer Simcha Rabinowitz, Nofet Tzufim, and R. Yosef Kapach.
However, there is a long list of traditionalists who did not (perhaps one should say: could not) accept that Rambam denied the existence of demons, and claimed that he believed in their existence. This list including recent figures such as R. Tzefanyah Arusi, R. Zvi Yehudah Kook and R. Shlomo Aviner.
Rambam’s most explicit denial of the existence of demons would seem to be found in his commentary to the Mishnah:
Amongst that which you should know is that the perfected philosophers do not believe in tzelamim, by which I mean talismanery, but scoff at them and at those who think that they possess efficacy… and I say this because I know that most people are seduced by this with great folly, and with similar things, and think that they are real—which is not so… and these are things that have received great publicity amongst the pagans, especially amongst the nation which is called the Sabians… and they wrote works dealings with the stars, and witchcraft… and demons, and soothsaying… (Commentary to the Mishnah, Avodah Zarah 4:7)
Some claim that this text is only rejecting conversing with demons, rather than rejecting the actual existence of demons per se. However, a careful reading does seem to make it clear that Rambam is saying that demons are not real.
Other evidence for Rambam’s denial of demons emerges from his discussion in the Guide of the prohibition against eating an animal’s blood was due to the belief that doing so has the effect of summoning demons who then become of assistance. In the course of this discussion, he writes as follows:
Know that this belief was widespread in the era of our teacher Moses. Many conducted themselves in accordance with it, and people were seduced by it. You find this written in the song of Ha’azinu: “They sacrificed to demons, not to God; to gods that they had never known.” The Sages explained the significance of the phrase “not to God,” when they said that the people not only worshipped actual beings but even imaginary ones. (Guide 3:46)
Further evidence for Rambam’s denial of the existence of demons comes from the fact that Rambam consistently either ignore the Talmudic references to demons or reinterprets the statements in such a way as to avoid accepting that demons exist. For example, the Talmud (Makkot 6b) refers to the possibility of someone being warned against a crime by a demon (which renders him liable for punishment if he nevertheless continues). But Rambam records this as someone hearing someone warning him but not having seen them. The Talmud (Berachot 3a) gives one reason why a person should not enter a ruined building as being due to danger from demons, but Rambam does not record any reason. The Talmud speaks of Adam giving birth to demons, but Rambam presents this as him giving birth to people who did not refine their intellects. There are countless other such examples.
Natan Slifkin, Wrestling with Demons: A History of Rabbinic Attitudes to Demons (NP: ZooTorah, 2011), 5-8.