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

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