Board Thread:Show-Related Questions and Answers/@comment-5989182-20130703185457/@comment-24062891-20131021200701

I think that because 1) Regina restored Daniel with someone elses' heart, and not Daniel's heart it was incompatible like an adverse reaction like an organ transplant that is not matched that's why I think it killed him.

My other hypothesis is 2) that Regina unknowingly used a morally bad/diseased heart, hence causing Daniel to be killed.

I will say both of my hypothesis 1) and 2) will go for the reason of why Frankenstein behaved badly after the transplant and what caused the death of Frankenstein as well.