Absolutely vile to the purest degree, Amanojaku are wicked beings. Ones which whisper humanity’s darkest thoughts into their hearts, to go against what they believe to be just and right. Corrupting and deceiving, tempting them to act on impulse and desire.
Though their true origins have been lost to the sands of time, and though they certainly have an oni-esque appearance (and even sometimes depicted as minor oni), there’s a good chance their mythos descended from the wicked shinto deities Amanozako, Amenosagume, and Amenowakahiko, who each have a similar degree of ‘role reversal’, ‘betrayal’, and ‘deception’ in their themes.
Though the Amanojaku existed long before written history in Japan, they’re commonly featured as an obstacle of sort for Buddhism - their true status as a lowly youkai which the gods trample underfoot, the just and righteous deities conquering over those who provoke wicked desires.
Overall, Amanojaku are liars, deceivers, and betrayers, walking corruption of spirituality, looking for nothing more than to twist the gullible and weak-willed around into doing their own bidding.
The most infamous legend of Amanojaku is that of the Melon Princess. A cautionary tale of why one should be cautious of strangers, and just who you trust - with a quite grisly end for the princess in question, who foolishly decided to trust one. The tale can be read here - though be aware, it can get graphic in the violence department!
TOUHOU-CANON LORE
Important to note - Amanojaku, while oni in myth, are not considered oni in Touhou project. This is because their dishonest nature is directly opposite of the honest nature of the oni. They are the exact opposite of a proper oni, an inversion, the reverse you could say.