I think Toni is really good at reading people and their intentions. Probably due to her growing up in foster care and having to assess pretty fast whether she could trust someone. Which is also why she loves Martha so much bc she pretty much only ever has peaceful intentions.
So when she meets Shelby, all her spidey senses tell her that Shelby is fake. That Shelby behind all the "love thy neighbour" attitude, was hiding hate and contempt. Which is why she reacts to viscerally to her getting close to Martha - the person she is meant to protect. The person that trusts too easily, while Toni is the person who has to be suspicious for both of them ("Why do I always have to fight your battles for you?" on the plane).
What she doesn't realize is that the hate that she feels bubbling underneath Shelbys surface is directed solely against Shelby herself.
Now we could ask ourselves why Toni never softens when she sees Shelby taking care of someone (even saving Tonis own life). Aside from the reasonable assumption that Shelby only does it bc she is a good little christian, I'm guessing there have been plenty of people who disappointed Toni, even if they showed real care for her before. Maybe a tiny Toni was welcomed into a foster family and treated well until something went wrong and she got angry and didnt know how to control it and maybe hurt one of the other kids and then was shipped off immediately again, no second chances. Maybe that was even in a family similar to what she imagines Shelbys family looks like.
But we can see Toni soften as soon as she sees a moment of vulnerability and honesty in Shelby, after Leah attacks her and she is forced to show everybody her dentures. When she realizes it actually hurts Shelby that she hates her ("just another reason to hate me"). If Shelby really hated Toni back - why would she be hurt by that?
And as soon as Shelby kisses her, everything makes sense all of the sudden.
The unnerving feeling of the outside of Shelby Goodkind not matching the inside, makes all the sense in world now.
Toni hadn't realized that the "fucked up shit" (as she said on their talk on the beach) Shelby was hiding wasn't a bad secret at all. Just Shelby thought it was a bad secret.
I do wish the story would've included a little bit more of Toni second-guessing herself about her first impression of Shelby. That maybe she would've realized sometimes that Shelby was honest when caring for other people. But alas the writhers thought the surprise was better without that kind of foreshadowing. I think it leaves a bit of a plot hole at how Toni could develop feelings for Shelby THAT fast. Like between their first kiss and their second kiss some stuff happened especially with Shelby (cutting of her hair e.g.) but that wasn't really that long of a time.
Sooo, are we thinking that maybe Toni started "liking" Shelby a little earlier? Maybe after the rage-induced destruction of their fort? Maybe even out of a little bit of guilt? We see them cooperating pretty well when looking for Fatin. So maybe Toni was warming up to Shelby (Shelby maybe was also giving off less 'hate beneath the surface' vibes bc of the time away from her family?). And then the mussles incident happens and Toni is double disappointed bc she had JUST started liking Shelby (kind of, secretly) and now she feels this real, true disgust coming from her. And thats just a punch in the gut.
And well - then the dentures happen and Shelby says 'fuck it' to risk and god, maybe bc she can't take Toni thinking she really hates her just after there had been a little peace. And also Toni is so fucking brave all the time, so maybe Shelby just wants that for herself as well.
