Vision did tell her though but Wanda shut him down, mid credit roll...It’s probably a deliberate irony that it’s Agatha who breaks through to Wanda to show her that she’s hurting people. It fits in with the pattern that it’s Agatha who helps Wanda the most in the last two episodes, when Vision and Monica couldn’t get through.
But it’s also part of the general sloppiness of the writing that apparently Vision could have fixed all this simply by breaking Wanda’s mind control on someone in front of Wanda. Apparently that’s all it took. But he never bothered to tell her, granted that Agatha was actively working to stop him from telling her.
Again the problem is not that Wanda did this but that the mystery format meant they couldn’t definitely say it until the end. Remember how “Norm” didn’t say who was controlling him or that “Agnes” told Vision that Wanda wouldn’t even let them think about leaving (which wasn’t exactly true)? All these red herrings got in the way of telling this very simple story of Wanda’s denial and her dilemma once she realizes what she’s done — and we never even find out exactly when she became aware, because we only see her from the outside.
The show is a tragedy of Wanda (tragic heroes do bad things), but it was disguised as a mystery, which made it more popular but less good than if we had actually seen it from Wanda’s POV and known what she knew.