Although it wasn't clear why Onyxia had to corrupt the politics of Stormwind City, she did a great job of it.
She took the form of a human –the noble Katrana Prestor – to take part in Stormwind City's politics. Because of things like her beauty and experience, she gained the appreciation and trust of the regent at the time, Bolvar Fordragon.
Half of the government decrees of Stormwind City were issued by her.
However, right when she was having fun playing her game, it was cut off. She was wanted and pursued by a horde of adventurers.
Although each time she died, it was only her projection, this sort of death really made her lose face. Moreover, she could also feel that as more of her projections were killed, her power also began to decrease.
And so, she abhorred this country. After the few waves of people she incited to kill Gorosh weren't able
to, she finally decided to take things into her own hands.