Lots of spoilers here…
MY theory is Emma destroys Regina by 1/ chopping down her apple tree 2/ Arresting her for malfeasance of office, extortion, tampering with evidence, any number of other offenses… Maybe Emma get the State Troopers / FBI to arrest her and take her out of Storybrooke thus ending her.
I think Gold / Rumples plotted Emma destroying Regina when he organized the curse in the beginning … He is just bringing his plan to fruition. SPOILER: BAE and Rumple was trying to find a way to turn him back to the loving father he was… so if he destroys all magic he supposedly could live a life that could allow him to be the man he started out being… no magic .. no evil.
BIG Spoiler: I do not believe that Emma attacks Regina with David's sword like the spoiler videos show. If she kills Regina she would / might inherit the evilness. I think the writers will come up with something better…. something we have not anticipated.
Nonnie