Karin is not a fangirl and that's why SS fans are pissed. Yes, she is a rather exotic sexual character, but also tragic and with a pure soul. She loves Sasuke, and for some reason, Sasuke too, has her in high regards that he, himself went to get her and he, himself sacrificed her which marked Sasuke's lowest point.
I've heard many try to make the case that Karin is not a fangirl. And I would say that if the only standard is about the first five chapters of the series, they yeah, she's beyond that. At least she knows something about him. To me, however, it's a distinction without a meaningful difference. With NS, we can look at concrete events and the overall development and understand why they like each other. We know it goes beyond what we saw from early Ino and Sakura. There is some actual depth there. With Hinata its similar. You can argue that maybe its mistaking hero worship for genuine love, but she sees something beyond, "He's so cool." Karin, I suppose might on par with Hinata, since the one thing that explains why is that recon where he saves her life. At least with Hinata, it's further justified and goes beyond one singular incident. And Naruto doesn't do anything to her that you would think would have her asking "Why'd I like this guy again?"
It's not expanded upon. It's not explored and no reasons arise from subsequent development that would explain why she still likes him. At this point, he saved her life and....she has this unwavering devotion that won't be stopped by even heinous murder attempts. So, even if you want to argue she loves him, there is zippo depth, it's explained entirely by retcon, and has the dubious distinction of having it's subsequent events actually argue against the notion that she has any reason or basis for it. So, she gets beyond chapter 1-10 Ino and Sakura. Whoop-de-doo. It's a real low hurdle to clear. In her limited defense, all of Team Not Seven suffers badly from this problem. Their motivations are so poorly explored that it's a masterful example of what not to do. So, I tend to believe the rumor that they were foisted on him by editors. It would explain a whole lot about how they've been treated.
It's kinda of like how we sit her and constantly wonder why Sakura still has any linger feelings for him. I have no idea why Karin clings to him. But at least we got to see active development and change with SS. I don't care for it as a pairing, but Kishimoto put effort to working with it. Rather than having to rest everything on a retcon. Most of the arguments I see for SK deal predominately with "whether" it will happen. I tend to have some agreement with that part, but the actual basis for why either party is interest is weak.
It is always bizarre to watch people trash SS in one breath while arguing for the virtues of SK. All the arguments of why SS is bad, why it can't happen, or why it shouldn't happen apply to SK.
She loves Sasuke, and for some reason, Sasuke too, has her in high regards that he, himself went to get her and he, himself sacrificed her which marked Sasuke's lowest point.
There is a vast different between holding someone in high regard and loving them. There are plenty of people I hold in high regard who I have no romantic interest in.
You know, the other day I needed a screw driver to fix my kids toy, so I, myself, went to go and get it. The screw is a specific size, so I need a specific one. It helped and I needed it. If my son had run off with it or thrown it it way, I'd try and stop him. I'd be upset if he succeed, but I don't want to marry my screw driver. Pretty much all her treatment by Sasuke prior to getting out of jail can just as easily explained the same way. He doesn't seek them out originally because he likes them, he does it because he deems them valuable tools. This idea is more consistent with his subsequent treatment of them (ditching two of them in Iron Country and attempting to kill the other). What do you do when you are done with a tool? You put it away. What do you do with one whose liability out weighs its value? You discard or dispose of it. Additionally, that portion is his descent into darkness. Unless people going to try and say then entire thing took place during the Danzou fight, I think it's entirely consistent (and it's about the only thing I'd call consistent when dealing with Team Not Seven) to have a guy who is falling into darkness (who is making a habit of ditching people) to treat people that way. It reflects so poorly on Sasuke, as I sure it's suppose to. I think it was tricksie last chapter who made this point, but even now Sasuke is still behaving this way.
The only thing that I can look at is the apology. I think it's hard to read deep into this as a sign of genuine affection because the whole thing comes across as an attempt to take a thorny issue and move it aside so he can get to other stuff. And, what's more, is that when I compare it to his leaving her bleeding on the forest floor, I know what I think is the more clear sign of how he thought of her from her introduction to that point. The apology looks more like the exception, especially since it comes after his (partial) change of heart. And like my point above, I think we are talking about standards so low that it's hard to be impressed by his actions.
Whereas with Karin, he went specifically for her, he gave her for some unknown reason the most OOC smile of his, that we've only seen prior Uchiha massacre [I mean, imagine that!], he awoke a new power from the desire and desperation to save her life and after sacrificing her, he used the most deep apologetic measures to show his sincere regret for what he has done to her.
If I were inclined to argue favorably for SK, I would hope to avoid words like "unknown" and "out of character" when trying to asses someone feelings. But ultimately, that's part of my point as to why the arguments for it are so unpersuasive. Even in NH, I don't have to use those words.
The problem with the "desperation to save her life" when it comes time to put up or shut up a short time later, he doesn't bother. She had value, he protected her. When she didn't he didn't even make the attempt.
And now that I think about it...haven't we, as a fandom, spent many many words downplaying the significance of the 181 "Thank you?" After all, how often did he do that? Subsequent events have made that unnecessary, but I fail to see how his apology is meaningfully different.
And besides the fact that they are forced to admit this fact with Karin, they're also forced to admit that Sakura holds Naruto in higher regards than she does Sasuke, and given that their only and strongest argument "But Sakura loves Sasuke!!!" is quite getting destroyed, it's hard... Imagine how hard it is.
Are they? Kishimoto has been very close to vest about the specific nature of her feelings for both Naruto and Sasuke, especially the latter. She cares deeply for both. I think he is moving in the NS direction, but that's based on events that don't necessarily do much to reveal Sakura feelings specifically. I don't think they have to admit what you claim, but I don't think that means much. I do think the writing has been on the wall for some time.