This is very similar to your later question; I will copy part of my answer there. But you really shouldn't be posting basically the same question more than once; you can edit your questions if you think you have found a way to ask it better.
First, she can be disliked without being hated, and without being a physical bully. She can be intentionally mean, disdainful or insulting. I don't know your setting, but an example is somebody informing the group of winning an award, and she responds "Congratulations, you made a hundred people unhappy doing it", or insults them or claims she thinks they are lying about it or cheated to get it.
Her flaw is that she is unhappy with her life in general, and other people's happiness feels unfair to her, they don't deserve happiness if she doesn't get any. Many unhappy people can't figure out why they are unhappy, and end up thinking it must be the fault of other people -- They are denying her happiness so they can have it themselves!
So she feels like others are cheating somehow, taking more than their share, making others unhappy so they can feel happy, and her insults (subconsciously to her) are a way of punishing them for taking something she doesn't know how to get.
But notice that flaw doesn't really apply to somebody else that isn't getting any happiness. If she thinks an innocent person is unfairly denied happiness like SHE is, then the innocent isn't guilty of "being happy." And in that case, your disliked character may have sympathy for somebody in the same boat as she is in, and it may be obvious to her that she can do something about it: she can be kind and sympathetic, and because she knows she is voluntarily providing any happiness that creates, she doesn't feel like the innocent stole happiness or took it at someone else's expense, she knows it was given freely because she gave it.
If you want, that can become a mutual ongoing exchange; the innocent can return the favor, and your disliked character could decide after some time to voluntarily extend a kindness to somebody else. she could doubt her assessment of them as "bad because they are happy", because she has learned that isn't always true, and sometimes giving away a kindness is how you make a friend, and friends can make you happy.
There is a character arc there, by the end of the story she might even apologize to somebody she insulted early on just because an apology is a kindness. She doesn't have to become a saint; but it would become clear she is on her way to stop being an ass.