how is it that when graham's сердце was taken out, he still remained good but when cora's сердце was out, she became evil?

The taking of your сердце doesn't make Ты good или evil. It only makes it so that Ты can't feel. Cora was always a scheming, ambitious woman, who ended up viewing Любовь as a weakness before she ever ended up taking her сердце out. She only grew еще scheming over time, which lead to her becoming an evil person.