Cora’s heart in the box glowed only red; there wasn’t even a small black spot like Snow’s in her heart. Cora put her heart away before any darkness had touched it, so the heart that Snow manipulated Regina into putting back was uncorrupted. The love Cora felt for her daughter in that moment was truly absolute love, untainted by anything.
So, not only did Snow cause the death of another human being, she caused the death of a pure heart. The heart was not corrupted. Had she simply returned Cora’s heart and not burned the candle, everything would have worked out perfectly for everyone.