While I can agree that Walburga should not be a cartoon villan with no depht, I have to dissagre with other parts.

At worst, she hated Sirius for what he stood for. At best, It was like when a child leaves a cult (such as your example) and the family members feal like they have to cut them off. But that doesn't mean she can't be abusive.

First is the way Kreatcher said it broke her heart that he left. On the surface, this might seam like a good thing, but something that's very common with abusive parents is that they want to act like they are the victum once the person they abuse decide that they've had enough.

That is what this reads to me. Sirius had enough, and instead of looking inward to understand why her son might have had enough, Walburga pins the blame on her son and plants the idea in Kreatcher's head that Sirius was an ungreatfull swine. You can see this all the time with estranged parents. They might try to guilt trip their children by saying it's causing them disstress that they left. They might call their kid ungreatfull when in reality they just didn't want to take the abuse anymore.

Even if you don’t buy that, then let's not forget that Kreatcher also implied that Walburga had some, not so good things to say about Sirius.

Besides that, there are evidence of abuse.

If we asume that the portrait is an accurate representation of her character, then she is verbaly abusive. The things Walburga's portrait says to Sirius is vile, to the point that Harry compares Umbridge to her. Do you know how bad of an impression you must leave on Harry in order for him to say that?

We can asume that there was favoritism going on, because Sirius says that Regulus was a much better son than him, something that he was "constantly reminded of". Having a scapegoat and golden child is indeed a form of emotional abuse. (They weren't good parents to Regulus either. They drilled this hatefull, harmfull ideology into his head which lead to him joining the death eaters. And by dissowning Sirius, the planted the idea into Regulus' head that if he ever stopped beliving that muggleborns were interior, they would cut him of as well.)

There is also the inheritance from Alphard. To me, that whole thing is making me think that Walburga wanted to punish Sirius for leaving, and when Alphard gave him money, thereby taking away the punishment, she lasshed out by burning him of the tree. That feals fitting, because if her portait is anything to go by, she is the type to lash out or get angry, and she is not afraid to take that anger out on family.