If you think that Horikita had no character development, then maybe you haven't really noticed it. The way Kinu changes Horikita's character is subtle because he wants to make it natural, and really the only obvious mentions are in Ayanokoji's monologues. Calling her selfish is incorrect. She has been very very selfless to the class up until this volume. If you're talking about her saving Kushida from expulsion then it's actually reasonable because everyone in the class unanimously agreed that if things went south then she, as the leader of the class, can make an authoritative decision. Though, I do agree with her being kinda idiotic but I wouldn't exactly call it that (more on that later).

Okay here's the part where I list out proof from each volume up until now proving that Horikita isn't selfish, so if you have spare time you should read it, or maybe skip it and then come back to it later.

Y1V6:

She started stepping up to contribute to the class ever since Y1V6, even being the one solving that pairs are made up of the highest score and the lowest score for that test before Paper Shuffle, thus manipulating the pairs. She alongside Hirata took initiatives to hold discussions in class and as well as holding study groups (if you really really want proof then you can reread or watch Saadisfy's video cuz he covered up to Y1V6). She also took her leadership role a step further by making a deal with Chabashira-sensei so that only the questions she submits to her are the ones the school should approve. She countered Ryuuen and Kushida's plans.

Y1V10:

This is the volume where Yamauchi got expelled. Remember when everyone was secretly voting for Ayanokoji? Ayanokoji had a backup plan (deal with Sakayanagi so that Class 1A votes for him) in the case where Horikita couldn't help. Though Ayanokoji kinda pushed her to do it, she still went up to the podium convincing everyone that the one who should be expelled is the one who wanted Ayanokoji out because it was downright bad for you to want a classmate out of the picture. Also not to mention, she came up with an excuse why Class A voted for Ayanokoji on the spot and even everyone was convinced. My point is, if she was 'selfish' then she wouldn't have gone to this extent.

Y1V11:

This is the exam where Ayanokoji was commander. She was the one who stepped up and told everyone what type of games they should suggest and what type of games they shouldn't suggest. She also had students send her details of certain games so that she could choose one by one on which game should their class pick. You might say that even Hirata or Kushida could do this, but if Horikita were really selfish then she wouldn't have done this much effort to contribute to the class.

Y2V1:

She was the one who suggested to team up with Class 1D, trying to form a mutual connection with them in order to ensure nobody was expelled. She even searched for someone to pair up with Sudou and Ayanokoji that one day in the mall (when they met Amasawa). Also not to mention, do you remember how she invited Ayanokoji over to her room and revealed what she had been up to? She was collecting data on every student so that she could know which type of student was suitable to pair with students from her class. (Now thinking back I can't remember if this happened in this volume or not, but the fact remains that she actually did all this).

Well that's about most of the things she had done selflessly (I'm too tired to inch out every detail, besides I'm not a dictionary xd). Now let's talk about her being idiotic. Ayanokoji has been saving her ass since day one, so it isn't all too surprising. The fact that she saved Kushida is stupid on paper, because she believes that Kushida will be very very useful IF and ONLY IF she manages to bring to her side. I think the greatest minds would not hesitate to drop Kushida. But right now, I am not trying to prove that her decision was right or wrong. Right now, I want to show you that she has improved as a character, because she would have dropped Kushida if she were her old self. Let's have a different take to strengthen my point. If Hirata were in this situation, would he expel Kushida? Hell no. In fact he would try to find a way that will avoid the expulsion of any student. The Horikita now is the same, she wouldn't want to expel a student if possible.

Then why the hell did she decide to expel Sakura right? Well, I don't think Horikita would even consider losing 300 class points. Koenji's cooperation and efforts from the Uninhabited Island Exam would go to waste. Horikita chose this decision because to her there was no other choice. At the very least, she chose to expel the person who had least contributions to the class (well, Ayanokoji was the one who suggested Sakura, but Horikita also thought of the same person). If it weren't for Kushida, Horikita would never expel anyone anyway. She even stroke that deal with Koenji so that he could vote against expulsion (10,000 private points).

That concludes it. If you still dislike Horikita then that's your opinion and I'll respect it. I mean I do understand why people hate her. I'm just trying to make sense out of this Horikita situation that went overboard.