People who are abused aren’t aware of the danger they’re in when they’re in an abusive relationship. They’d think the better of their abusers. If you’ve seen accounts of their tales and have worked in a non profit organisation for domestic abuse. They’re usually the ones who think “it’s fine, he doesn’t really mean it, he’ll get better someday, I’m here for him” and just let it happen.
I know how abuse works. I know that people who are abused only try to see the good. A lot of the times when people go back to their abusers it's because that's all they know. They're used to the routine of it. They're minds have often been molded to see that they are the ones at fault and that they wouldn't be abused if they hadn't done something to deserve it. After being abused they start to believe that the abuser isn't abusing them and that their abuser is the only person capable of loving them and that other people would not put up with their 'bullshit' like their abuser does. I was pointing out that Nagisa truly believes that Shichiku didn't mean to abuse him but now there is always a chance that Shichiku could lose his temper and hurt Nagisa again. From reading the manga, I think that was the first time Shichiku had lost his temper with Nagisa, but it definitely was not the first time he had abused someone. I know Nagisa has hope for a nice, healthy relationship but I don't think it is possible. Sure in the comic Shichiku may be able to change but realistically speaking, he would probably still have fits of anger here and there in which he hurts Nagisa.
I've been through and seen both mental and physical abuse both firsthand and secondhand several times throughout my life. My father and uncle(two of them to be exact), my ex's father, my ex, as well as my best friend's father were all abusers. And I've taken psychology classes to further extend my knowledge of the matter. People who abuse others actually have no self-confidence and they abuse in order to gain power over people. Often times people who abuse others usually only get worse each time they hurt someone. I'm a strong believer in giving people a second chance but people don't change. IMy know that sometimes a second chance is the last thing someone needs after hurting you, but I also know that once you're abused it's easy to believe it was because you had done something(because you have been conditioned to think that way) and that with this chance your abuser will stop. My homicide professor even states that the core of one's being does not change. Sure, people can stop abusing others but deep down every once in a while they still feel the need to abuse. Even if it's been years upon years of them no longer abusing people, at some point they may lose control and revert back to their old ways. You can change how you act and who you are but your original core is still there.
I don't think Nagisa is fucked up in the head nor is he masochistic.
I do think he cared deeply for Shichiku and that his love for Shichiku blinded him from what was going on.
I think that Nagisa is a kind, gentle person and he is extremely loving to those he cares about. But sometimes, people like that are naïve and love can actually cause you to overlook things important like the abuse he went through.
His love just simply allowed him to see Shichiku as what he wanted in a lover and despite being hurt, I think he overlooked the abuse. But this could also be due to the fact he has a bit of trauma in his past so he may have felt that It's okay to be someone's punching bag, even if it caused him a great deal of pain. He definitely needs to see a therapist and get some help.
Nagisa is a bit too forgiving in my opinion. But also, Shichiku had a psychological break in his sanity due to the stress and self-hatred as well as his mentality that he was never good enough. So in reality, Shichiku didn't really mean to abuse Nagisa and I sincerely believe that if he could think straight he wouldn't have hurt Nagisa. In the emd, Shichiku is more of a cheater rather than an abuser but I do think he has seen the error of his ways. As for Nagisa, he was only focused on the good and he focused on his feelings rather than the abuse. In a way, I think his subconscious may have blocked it out a bit in the end.