The question is, if Zelena wasn't carrying an innocent child, she would have killed her?
She would have killed her sister, right there, in New York, for tricking two of the most important persons of her life?
Probably, no. Because now she knows best. Still, the fire comes so easily to the tips of her fingers. She stops in time, in the Round Table room, because she won't harm an innocent.
No more.
Zelena is smiling, thinking she has won – her sister could have killed Mary Margaret, and they stayed alone for half an hour.
She holds the fire inside.