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

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