We may only be a few episodes into Once Upon A Time Season 5, but things are already pretty intense, and it seems like it's just going to get worse from here. It only took six weeks for Emma to give into her new role as the Dark One completely, and while she may have erased everyone's memories of what took place during their brief sojourn to Camelot in an attempt to save her, whatever happened there was enough to push the former Savior over the edge and into complete darkness. Regina is the reason she's in this predicament to begin with, considering Emma became the Dark One to preserve the former Evil Queen's chance at a happy ending. It's also Regina that Emma entrusted with the task of saving her, at least initially, handing over her dagger — her biggest source of power and the key to controlling her entire existence. The problem is, at least from what we've seen so far, Regina hasn't been doing a very good job.
Believe me, it breaks my heart to say it as much as it probably does yours to read it. I'm not only a staunch Evil Regal, but also a ride or die Swan Queen 'shipper, so it pains me to see the barriers that Emma and Regina worked so hard to break down being rebuilt with every slight, every disappointment and act of vengeance. One of the first things Emma said to Regina upon seeing her for the first time after taking on the darkness for her was, "I saved you, now save me." Regina is finally ready to step up and be the savior Storybrooke needs, but she'll need to do a lot more if she's going to be the savior Emma needs, and that's the most important role of all.
Without a doubt, Regina is all too aware of just how big a sacrifice Emma has made for her, and with all that we know about Regina, it's likely that she's not quite sure what to do with that realization. For her entire life, she's been known as the Evil Queen. Every time she's thought she might have something good, it's backfired. People used her, manipulated her, betrayed her, left her. All she knows is that other people disappoint her, and now that someone not only hasn't done that, but has put her first and followed through on a promise, I don't really think she knows what to do with it.
Of course, this doesn't dismiss her behavior in Camelot so far. While Regina does move to protect Emma by silencing her with the dagger and presenting herself as the Savior to King Arthur, she undoes that good deed not long after. Emma is initially thankful that Regina is steadfast in her dedication to keeping the new Dark One from using her dark magic, but when Robin is stabbed by Percival after he discovers Regina's true identity, Regina forgets that vow and begs Emma to heal him. It's understandable that she wouldn't want Robin, who she still believes is her soul mate because of Tink's whacked out prophecy 30+ years ago, to die. However, it seems rather uncharacteristic of her to completely disregard Emma's well-being — and in fact the entire fate of her soul — to ensure Robin's. Sure, she gave Emma a choice, but Emma became the Dark One without a second thought for her. Would she really turn her back on Regina now? I don't think so, and Regina knew it.
Back in present day Storybrooke, all we know is that something bad enough happened in Camelot that Emma not only embraced her darkness, but created a curse to erase everyone's memories of what exactly that was. She feels let down, and while it hasn't explicitly been said by whom, it's pretty clear that it's Regina. No doubt what transpired between them that ultimately led to Emma's current state was pretty serious, and from the clues we have so far, it seems that Regina let Emma down yet again, and finally, enough was enough. After all, how else do you explain Emma's insistence that she's done fixing Regina's problems, or her cutting remark about Regina's lack of regard for human life? Whatever went down between them was serious, and seemingly irreparable. What kind of Savior is Regina if the one person who needs her most is the one she just won't put first?
I don't for a second believe that Regina let Emma down purposely, or that she was even aware of the realities of her own actions or Emma's feelings about them. If the showrunners do indeed intend for Swan Queen to be endgame (and let's be honest, is there any other viable option?), then it seems pretty obvious what's going on here. It all comes down to Emma's inability to vocalize her feelings, preferring instead to believe that her actions — and honestly, what action speaks louder than sacrificing your entire soul for someone? — can do the trick. Regina is so focused on this pre-written form of "happiness" that it's now almost more about getting what she believes all reformed heroes deserve than really examining if she actually wants to be with Robin. Thinking about Emma as a viable option in any way hasn't come into her mind, not only because she still believes Tinkerbelle's pixie dust prediction, but because the thought is likely too overwhelming and terrifying to actually entertain.
The truth is, we all do terrible things to people we care about without realizing it sometimes. But Emma hasn't made it easy for Regina to redeem herself this time. She's taken Regina's memories of the ways in which she let the new Dark One down, making it impossible for her to fully grasp the seriousness of the situation or figure out what she can do to rectify it. Emma seems to have lost all hope; she feels let down by the people she cares about, and Regina is at the top of that list. But part of her still believes Regina might come through in the end, which is why she's taunting her, goading her into stepping up to the plate and actually being the savior she claims herself to be. How she can do that and what the price will be is unknown to Regina just now, but she'll only begin to find the answers if she starts being honest with herself about who she loves — and the one person in particular who loves her back more than any others. I believe in her. After all, who's better matched than the Savior and the Evil Queen?