I think she was angry when she wrote it and then the next day, when she saw it on the table she was sorry and was trying to make it better/make it never have happened.
She tried to make amends by scribbling it out, but when the OP asked her about it she hadn't fully processed her feelings. She felt reminded that she had done a bad thing at the exact moment that she was trying to fix it. Her head was in the "fixing it" phase and not yet in the "able to talk about it" phase.
And the result was her anger because she felt she was doing a good thing to make up for her behaviour and the OP reminded her of her earlier poor behaviour.
These are big emotions for her to process.
I would have thanked her for scribbling it out and said something like, "Thank you for removing the words. When I saw what you had written I felt very sad and upset, but I can see now that you are trying to make up for it and I feel happy that you are doing that".
I use the formula "when you do x, I feel y" a lot because both are true and it helps children to learn that others can have difficult feelings too.
I would also focus on the good behaviour and ignore the bad and start saying things like, "I really like it when you help out; I can see that see you are working hard and I feel really proud of you; when you are playing quietly I think that's lovely."
If children get more attention when they misbehave than when they behave, they will misbehave to get the attention, even if that means being shouted at.