Board Thread:Show-Related Questions and Answers/@comment-24304025-20141219181020/@comment-91424-20141219184543

It is never stated why Zelena couldn't kill Emma. Although it could have something to do with the second curse. Remember, at this point everyone believe that Emma is still the savior. During the first curse, in "An Apple Red As Blood", we found out that if the savior is killed, the curse will be broken. This is what Regina and Gold said:

Regina: I think I'd rather just get rid of her.

Mr. Gold: Whoo! Oh, well. You're going to have to be quite creative. We both know the repercussions of killing Miss Swan.

Together: The curse would be broken.

Regina: That's because you designed it that way.

Of course, we do not know if the second curse works the same way, so this is speculation.