I've been thinking a lot about how the Miraculous works for its user, and why certain people are CHOSEN as holders of the different Miraculouses. They are "chosen" in a sense by the nature of the specific Miraculous itself, as a power object which embodies archetypal ideas. But it's necessary for wise men like Master Fu to help a Kwami find its ideal partner, a user very specific to that particular Miraculous, according to their individual personality type.
I came to this conclusion by thinking about exactly what kind of powers it is that a Miraculous confers on its user. That is specific to each Miraculous, but in general we can say that the way a Miraculous works is to enhance the specific abilities of a user who is "in tune" with it, and no other Miraculous. It makes Marinette into a better version of who she already is. I thought about how she's kind of dorky, unsure of herself, and a klutz as Marinette, but as Ladybug she's stronger, faster, more agile, more aware of everything around her, and more confident. In some ways she even seems a little smarter than she normally is. That is a potential that Marinette always possessed, but the Miraculous enhances all of her natural abilities and potential and transforms her into her ultimate self. During her transformation sequence, she performs this quick and graceful little series of dance moves that seem like a cross between some kind of gymnastics and a ballet routine, and she executes it flawlessly, because the Miraculous instantly brings out her best potential and multiplies it to the Nth degree. The same is true of Cat Noir, whose transformation sequence seems to be based on some sort of martial arts moves, and he ends it in a fighting stance.
Master Fu had made some comments about what would happen if a Miraculous fell into the wrong hands, and I think he was talking about something more specific to each Miraculous than to a "good guy vs. bad guy" kind of thing. While the RIGHT Miraculous (and the right Kwami) brings out the BEST potential of the RIGHT user (that is, a "Chosen One"), the WRONG Miraculous in the hands of the WRONG user would do exactly the opposite -- that is, to bring out the WORST potential of the negative elements of someone's personality.
And this goes right to the story of Hawk Moth. In the RIGHT hands, the Butterfly Miraculous could be a wonderful thing, and bring out the best potential of the RIGHT user. But in the wrong hands, it has exactly the opposite effect, multiplying all of the worst, most negative aspects of Gabriel Agreste and making him a true supervillain.
Thinking about that and about the new villain, Le Paon, about to be introduced into the story -- who looks very much like a dark, evil version of Mrs. Agreste, I think I have some idea of what happened. The Butterfly Miraculous was never intended for Gabriel Agreste, for his personality was unsuited to it. I believe that Gabriel was the former holder of the Peacock Miraculous, based on the relationship of husband and wife, and the fact that there is no better metaphor in the human world for a peacock than a world-famous fashion designer. As the holder of the Peacock Miraculous, it would have brought out the best in Gabriel, and made him into a superhero -- and I believe the same is true of Olga Alexandrova (a.k.a. Mrs. Agreste) as the holder of the Butterfly Miraculous. With her as its user, she also would have been a superhero.
Furthermore, based on how well Ladybug and Cat Noir work together as superheroes, there seems to be some evidence that certain Miraculouses are complementary and that when their users team up, they are that much more effective, with each's innate skills and abilities fitting together like interlocking puzzle pieces. That is probably particularly true if the two users have a love connection (whether they may be consciously aware of it or not), because that bond will allow them to sense exactly where the other is lacking and "fill the gaps" in the other's abilities, and that love will also keep them both true to themselves and their sense of purpose.
BUT somehow, the users of the Butterfly and Peacock Miraculouses SWAPPED Miraculouses, and that is not allowed. While the Butterfly Miraculous would make Olga an effective superhero, and the same applies to Gabriel as the holder of the Peacock Miraculous, the opposite is NOT true. When the users swapped, the unfamiliar Miraculouses to which their personalities were NOT in sync, instead brings out the WORST negative potential in each of them. Thus, Gabriel becomes Hawk Moth, and Olga becomes Le Paon, representing the absolute worst in each of them.
And Ladybug and Cat Noir should absolutely NOT EVER swap Miraculouses, because that would be a recipe for disaster.