I have been thinking that the APPLE TREE that Regina so lovingly tends in FTL and here in StoryBrooke some how holds her source of power or magic. I think EMMA is actually attacking the tree and chopping it down in the scene where EMMA swings the sword. I do not think Emma would really attack REGINA and kill her as there would be consequences…. either in FTL where she takes on Regina’s evil nature OR in SB where she would be held for murder… there is NO WAY she could tell a judge that it was self defense.
I think it is against Emma’s nature for her to murder someone… as I have said before there IS NO EVIDENCE that FTL existed/ Snow-MM and PC-David are her parents.