There is a pool of possible motivations that a bad fairy may draw from – they don’t all have the exact same reasons but they do come from the same source.
a fast way to get power over someone is to make that person feel bad. When you feel bad you are less likely to recognize manipulation. “I heard you were a total slut! …. you should totally do something about that!”
it’s Wet Blanket 202. “Oh, you accomplished something great. But these people still think you’re horrible.” (I sometimes think of this as Pagan culture, the common denominator. Everyone has something insightful to quote about how much someone else thinks you suck.)
It’s secondary manipulation; they still want control over you because they hate how you’re changing the status quo.
In the rarest of cases:
Someone just took in lot of poison on your behalf and now they just want the antidote. This is the absolute rarest of cases; before you even consider that that might be the situation, make sure you know this person is otherwise a consistently good person in all other aspects.