Kawaii

(Literally got them from the villains wikia)

A fairy doing genocide

Fairies are sapient beings with bug-like wings, apart from angels who's wings resemble more like doves and birds. Fairies are either small like a bug, or just the size of humans. Usually their magic are elemental in a certain way for example, some fairies have powers linked to plants, some linked to ice and more. Although fairies are classified as species and a lot of fairies were born fairies, it has been noted that under some conditions, a human or another creature has become fairies and vice versa. It should be noted that both small and large fairies can do and will do some serious crimes. Smaller ones tend to murder creatures that they thought are to be inferior, which includes monsters, goblins, trolls, and others. Sometimes even humans.
Larger ones are more self explanatory as they are more associated to rulers and make sure oppression continues, and democracy is far away. It was probably the reason why after the roman republic, there was no democracy. They've might also have called for genocide with multiple races. They could be said to be on multiple planets.

Not much is known about their history, but somehow they came about and started murdering races one by one and trying to establish their dominance on others. This is where fairy hunters came in. Either voluntarily or paid, many fought against fairies destroying fairy groups trying to destroy many races protecting others. It was sometimes dangerous as there were powerful fairies trying to kill them. Many people from many different species including humans and youkai became fairy hunters and thanked the fairy hunters for their service.
However fairies were smart, they decided to go to the core of this. Many started establishing kingdoms known as fairy kingdoms which were absolute monarchies and sent fairies to influence the other nations. Eventually, those nations became under control by fairies. And what started happening is that races started going against each other. This was a way so fairies could gain dominance as other races divided against others. The kingdom under fairy control became oppressive and corrupt, just like the fairy kingdoms.

Fairies also made sure to put a good picture of themselves, this is why many are unaware of the oppression of fairies. In order to establish further control, fairies also made sure to put the guy in charge of a non fairy nation with absolute control. That way no one could oppose them and could just control one person rather than multiple ones. Fairies made sure democracy was long gone so nobody could oppose them. Many even tried to play god and would state that without them, there would be chaos and destruction of even nature everywhere. This way, everyone would only think good about them rather than oppose them.

With large influence on a large expanse in the universe, fairies started their genocide. Fairies made sure these races were looked upon as evil so nobody would even care if they were killed. An example of this was goblins

Another example were monsters, these were especially targeted by fairies which's why anti winx welcomes them.

A monster being killed.

There are so many more races that fairies want to kill that it would take too long to list. There genocide was so extensive that they were even able to commit genocide to extinction. On planet of Andros, fairies have been said to have fully killed the Melusina to extinction. The planet till today is still under control by fairies. Fairies have also made every race they hate be depicted as evil which in truth many are good people.

Fairy hunters during those dark times were despised and sometimes have to fight against species that aren't being destroyed by fairies yet. As fairies will kill a group of species before taking on the next group. During those times, many were on the run while fairies influenced the public to start killing more and more fairy hunters. Eventually fairy hunters had enough and decided to do something about it.

Some fairy hunters would start getting people to start revolution against their governments into democracies as they were the first to adopt that. Others would even try creating their own nation against those stupid fairy monarchies and autocracies and turn it into a democracy of inclusiveness. Still others try to bring light to those races oppressed and attacked by fairies and would gain influence. It is especially becoming much of a success today.