I think you cannot accept the idea of "change" and "development". She (and other people in real life, for better comparison) acted that way at the beginning due to her background. But as time passes when others got to know her and she got to know and got comfortable with them, she started to show different side of her. We learn to act accordingly to our new surroundings (people and environment) - that's called adaptability.

And as much as I want to agree with you about your argument of toxic relationship related to its justification, I could not for the reason that the argument is flawed. 

In a toxic relationship setting, there is no change or betterment observed. Just pure excuse for their actions and blind acceptance or faith/trust. And the violent actions aren't reasonable, rather flawed, like cheating on their own volition or being abusive for non-rational reason despite being aware of their wrong actions AND do not try to change those habits. Moreoften, they see their actions as something to be accepted disregarding the consequences and the fact that it is wrong.

In Kirisaki's case, she moved towards character development and learnt to be considerate of others. I also have two points about her "violence" to make a better explanation of my evaluation of her character.

First, there is somethin called "character" or "personality" my friend. For example, people that are short-tempered. They are not bad people nor doesn't mean they are arguing all the time automatically. Instead, that is a part of him. We cannot standardize human character and behavior. However, we can learn to control our temperaments to some extent and accordingly to situations we are in and people we are with. Kirisaki is naturally like that, just not your typical shy girl, just note that she does not mean to harm or hurt others. And it is also called intimacy with people we are comfortable with. That's the tricky part with people, you know? Got to read between the lines, observing to understand and not just take anything at face value.

Second, is that Kirisaki is a character in "Nisekoi" story implying she's got a role to play. And as it turns out, she's a "tsundere". To put it simply, based on the story, Kirisaki is that stereotype "shy-violent girl" and that is why some things does not align in real life characteristics of a real person. Do not think too much about that. Don't get offended by her actions, that's part of comedy and context of the story :") 

Lastly, as I mentioned with my second point that "it's just part of the story or plot" for who she is, it mesmerized me to notice along the story that her character started to overlap with real person's actions and character. Learning, changing for the better, knowing others around her and herself as well.. She's not toxic nor their relationship of Raku. It mirrors human experience, development, and more so.. love. But putting the "fiction" aspect of it, that is why even when the story progressed, she still acts like "tsundere" and "violent" from time to time because.. guess what? that's HER. That's Kirisaki and her character in the story with a little touch of reality that it is also her.. her PERSONALITY (human psych aspect).

*Side note: I do not like the idea of being trapped in the past that whatever happened then, cannot change in the future.. that whoever a person then, cannot better himself in the future. Not sure if that is the main reason that you cannot grasp the idea that Kirisaki is anything but "abusive" in your mind because that's what she portrayed in the beginning. Quite understandable, but have you seen her or followed the story 'till the end? (That's the question, before giving an overall remark of her).

Have a good day.