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The thing is, in Heroes of Might and Magic V, demons don't differ much from other monsters that just want to kill and destroy. In Heroes of Might and Magic III, however, they are more intelligent, cunning, and won't just kill you outright; they might torment you first. In Heroes of Might and Magic V, they are chaotic and stupid, meaning they don't differ much from orcs.
Conclusion (in my opinion): Perhaps many people are used to chaotic demons whose purpose is to destroy the whole world, plunging it into fire, and who mainly use brute force and are just cannon fodder. I, however, prefer cunning and insidious tempters, masters of lies and deceit, whose main weapon is not physical strength, but intelligence. In Heroes of Might and Magic V, demons are more like the first option than the second.
(I remember the plot with Biara, but overall, the appearance of the demons doesn't give the impression that they possess great intelligence, which is usually attributed to orcs).