I see a lot of people telling that Robbie rotten is a fairy so I got a lil theory about that :)
So my idea is that Robbie was actually a superhero n' was one of the heroes (n°8 or n°9 idk -) and helped lazytown for not being lazy. But one day, it has a rumor that Robbie did something that he shouldn't (like illegal for the superheroes/fairys or something -) and got call guilty by the judge. And for his punishment, they (the officers or idk who-) took his wings so he can't fly anymore n' stopped being called a "superhero". That's why he became a villain and want sportacus to leave lazytown forever, for his revenge.
(I still don't know why he's scared of height but anyway-)
It's just a theory that I made lol
(also, English is not my first language so sorry if I made some mistakes while writing -)