This story was actually pretty sweet and a great redemption for Kan. I've read some manga ...

This story was actually pretty sweet and a great redemption for Kan. I've read some manga where the abuser more or less gets away with his abuse. The victim leaves, finds their own love. And then the abuser finds their love too with no real consequence or regret over their actions. Heck, sometimes the victim even forgives them.
What I like here is that it shows the toll the abuse took on Kan as well. While obviously his circumstances aren't the same as other abusers in that he wasn't just genuinely a huge jerk sociopath, his backstory wasn't treated as something that absolves him of his abuse either. Kan was being abused at work, and in turn, took it out on Yumi. He is aware of it. Yumi is aware of it. Knowing he won't stop and things won't go back to being the same is what ultimately makes him finally let Yumi leave. And Yumi does as well, realizing there's nothing he can do and that he doesn't need to stay and take it.
Afterwards, Kan is left with guilt and believes he doesn't deserve love. Honestly, the man needs therapy to undo all that abuse heaped on him and to undo his abusive habits. Instead, Kan created his own coping mechanism by taking it all inward, redirecting his anger onto himself instead of Shuuna, as shown when he chooses to hurt himself than to punch Shuuna.
Shuuna recognizes this is no way to go and tries to put him at ease because he doesn't want Kan to live the rest of his life walking on eggshells either.
This is what makes the ending more satisfying. Because ultimately it's just Shuuna's reassurance that he'll always be there to support him that helps Kan move on. Shuuna thinks Kan should try talking to Yumi, maybe get closure by having him forgive him. But Kan knows that's not good and it's just a cop-out. He doesn't want to forget what he did because he genuinely wants to atone for it.
And I love that the mangaka showed that. Yumi, as well, is not required to forgive Kan. No one is required to forgive their abuser just to put their mind at ease. That's why his final peace treaty, a photo album for Kan to store the photos in rather than his wall, is all the more poignant. It's Yumi's way of saying he's moved on without directly giving words of forgiveness, and it's a way for Kan to keep holding onto the memories without letting it burden him.
Kan won't forget what he did, but his photos are now in the same spot as all his photos with Shuuna, his loved on, and in order. So he can see his growth and see that he's not some horrible human being beyond redemption. He's content to know Yumi is happy, Yumi is content to know he's finally happy. And that's all they need.
The mangaka was able to show the complexities and nuances in Kan and Yumi's relationship without condemning either one and without hand-waving away the abuse. Because not everything is black or white. In the end, this was a very sweet read.