It is not surprising that Magneto loses his mind after being forced to kill mutants he swear to defend. Is the worst sin anyone could commit against him.
Anyone who has any part in it or try to get in his way will have to face his fury. Which means bad news for Emma.
Now about him coming back to be a villain or not I doubt it. There is no more space today for the Magneto who once tried to dominate the world or enslave humankind or make mutants the dominant race on the planet but he will never be a hero in the same way. Modern Magneto is destined to walk this gray path.