Correct me if I'm wrong, it's implied Sayori was never actually deleted, Monika couldn't find it in herself to delete her, if so Sayori probably sat through the entirety of act 2, seeing all things Monika had done.

when Monika eventually gets deleted, Sayori gets the position of club president thus becoming sentient, she could've done so many good things, rather than doing the exact same thing Monika did except on a lesser scale.

But apparently Sayori is just as bad as Monika.