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

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