Why are they making Regina evil again?

I've heard she will be going back to her evil ways. It just doesn't make any sense! She's worked so hard throughout season 3 at being the hero so why would they have her go back to her evil ways and just waste all that hard work she's done at being the person Henry wants her to be? I love seeing evil Regina but I prefer seeing it in the flashback scenes. This better be temporary and she better not go 100% bad and the heroes better not go "oh she's evil again? Ok let's go back to killing her" and instead try to help her come back to her senses. This is my only complaint of season 4 is how they're gonna make Regina go bad again. Again this better be temporary and she and Snow better still be on good terms and she better still be willing to accept Emma as a friend after seeing that heartfelt scene in "Breaking Glass."