You can read Nagatoro in different layers.
The first, surface one is what it's marketed as: sadist bitch abuses innocent nerd, hijinks and fanservice ensue, no problem here.
The second one is the genuine love story reading: you completely ignore the bullying, zone out the violence, and you start to notice all the (not so) small details that show Nagatoro genuinely appreciates time spent with a guy she absolutely ADORES. The panels of her honest emotion become prevalent over the sadistic powerplay. The sleeping painting scene is an obvious example. The kiss is obviously honest, her sticking out her tongue accentuates its power. Smaller hints riddle the different chapters.
The third one is the anxiety reading: feeding off the previous layer, you incorporate back all the violence and bullying and give them purpose. Nagatoro is desperate for attention from a person who is capable of empathy, the only one she can find among a sea of antagonistic assholes. She carries the contradiction of trying to show affection, but using the wrong tools. The moments where she nearly loses control of the situation are not just visible, but striking. Now, the scenes where she acts like a bully reveal their contradictory, hidden layer. When she smiles as she abuses him, it's not just sadism, it's genuine happiness, a happiness she constantly fears she will lose forever. The panel in the first chapter where the four girls look at the scattered pages of his comic has an interesting touch. She doesn't laugh with the other girls, instead she examines his effort and carefully waits for the others to leave. She wants him so much she cannot afford to hurt him too much by involving her competitors. The cafeteria scene is also emblematic. Nagatoro visibly struggles to contain her crippling anxiety all throughout the hidden quarrel with her female friencds and is ultimately saved by Senpai, which brings us to the last (as far as I can see) layer, in the next post.
The fourth one, which is the darkest one, is the manipulation reading: Senpai is not innocent. Whether unconsciously or not, he takes
advantage of Nagatoro's fears, loneliness, expectations explained in the above layer. She abuses him and he keeps coming back. Why? Because he instinctively feels her attachment to him and exploits her fears to win her heart. The situation is never one where Nagatoro is in a safe space and Senpai is invited into a dangerous terrain. It's always insecure for Nagatoro and, by proxy of being used to rejection and pain, safe for Senpai. Going back to the cafeteria scene, Senpai doesn't like the cafeteria, but goes anyway because he is used to bullying by now. Nagatoro, however, is not prepared for a situation where she has to compete with the girls. Senpai notices the situation is getting too tense for her to handle and bails her out, attaching her more to himself as a reliable person. Senpai adapts, notice how he learns and becomes a better seducer in the course of each chapter. Now, again, I'm not trying to portray Senpai as consciously manipulative, there are no signs of this in the manga (however, you can play a fun game by looking the panels where one of his eyes is hidden behind glasses or off-panel, hide the visible eye and see how his expression changes). However, what Senpai IS capable of is taking control of a situation where he is NOT in power and exploiting Nagatoro's insecurities. This is obvious by Chapter 18. There, he's not just in her house, he freely asserts dominance over her to the point where she has to put extra effort cheating. He allows her to cheat and win, he baits her so she doesn't lose interest and, in the middle of her desperate attempts of abusive behavior, she completely gets played by him. Senpai is slowly but surely capturing Nagatoro's heart using his mastery over anxiety, both his and hers.
