is sun wukong really a demon or is the term ''demon'' used as an allegorical term for his attitude and misdeeds?
Well anon, while I definitely can't claim any expertise on the Chinese concept of "demon," I will say from what I know that "demons" in Chinese culture do seem to be quite different from those from a Euroamerican background.
Mainly, "demon" in Euroamerican culture generally seems to refer to a strictly evil entity--traditionally originating as fallen angel--whose primary goal is to lead humans astray so that they'll sin and be doomed to an eternity in hell. They stand as the polar opposite of the angels and heaven, and while this kind of demon has been depicted in a wide variety of forms, their goals are unchanging.
A "demon" in Chinese mythology, on the other hand, is a "yaoguai," or an entity that's perhaps more akin to the English term "monster" in that while they certainly can be--and as demonstrated by Journey to the West often are-- malevolent to humans, as a group they seem far less interested in leading humans into a life of sin and far more interested in fulfilling their own desires, as well as in cultivating Taoist power so that they can attain a humanoid and immortal form. Yaoguai seem to have a wide variety of origins--to give a sample, the Lady White Bone originated from a human skeleton, Sun Wukong came from a giant stone, and Red Boy was the progeny of two married yaoguai--and they also seem to lead a wide variety of different kinds of lives (some are married with kids, some are warlords, some just keep to themselves, etc.). That said, there does seem to be a commonality between many of them in terms of living in mountain caves and having a distinct preference for human flesh. Even so, there seems to exist a much more fluid boundary between animals, yaoguai, humans, and even gods than Euroamerican thinking usually permits and that seems to be significantly based on how one chooses between the possibilities inherent in one's life. It is this fluidity that makes Sun Wukong such an interesting case; you could argue, after all, that he is both monkey, human, demon, god, and buddha all at the same time.
Here's a few samples from professor of Chinese literature Hongmei Sun's analysis of Sun Wukong in her pretty excellent work Transforming Monkey that go over the ambiguous character of the monkey king:
And finally, as Sun notes, Andrew Plaks argues that “the characteristic Chinese solution to the problem of duality ‘consists in the conception of a universe with neither beginning nor end, neither eschatological nor teleological purpose, within which all of the conceivable opposites of sensory and intellectual experience are contained, such that the poles of duality emerge as complementary within the intelligibility of the whole.’ This argument about the Chinese concept of complementary duality provides an interesting explanation for the coexistence of contradictions in the narrative. It may also count as one of the cultural situation ‘generative of ambivalence and contradiction’ that folklorist Laura Makarius discusses. The concept of complementary duality in Chinese culture certainly helps explain the fundamental ambiguity regarding the teachings in the journey, the most famous being the merging boundary between god and demon.”
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So in conclusion--and keeping in mind as always that I'm in no way an authority on this & that people who know more should feel free to add on to what I've said here or correct me if I'm wrong--I think you could certainly describe many aspects of Sun Wukong (including much of his behavior) as demonic, but that this only counts for a part of his multifaceted character. He may be a yaoguai, after all, but he is also the Buddha Victorious in Strife.
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Also (and in hopes of not being too annoying lol) I’d like to tag the ever-interesting @journeytothewestresearch to this to see if they might have something to add about the nature of Sun Wukong specifically or yaoguai in general.