Emma siding with Regina, and taking the darker path.
I was freaking out about this scene, and I didn’t realize this until I was giffing, but look at Regina’s face in that last frame. She’s smiling, and she seems victorious because Emma chose her over Snow (which is huge, right?). But, look closer. Because her smile drops. She grimaces at the end. It should be a huge victory for her, but instead it’s not; it’s a realization.
She is seeing Emma, someone that is portrayed as the purest person there is, the savior, but she’s siding with Regina. She’s choosing darkness and “evil” over light and “goodness”. And she thinks it’s her fault. She’s been told and shown over and over that she corrupts anything that is near her, and that is what is happening with Emma. This is a reinforcement of everything negative she’s ever believed about herself.
In addition, it’s her losing the last bit of hope she had for her own redemption. She’s just heard that Henry, who rejected everything dark so vehemently, is violent. Emma was hope for her, a lightness in her life, and more importantly, her son’s life. But she’s also begun to genuinely care about Emma (as at least a co-parent), and she’s seen too many people she’s cared about lost to darkness (either of their own path, or through a relationship with her), and she doesn’t want that. Not for Emma, too.