Okay, this is definitely not what anyone would call a healthy relationship, Misaki is assaulted by Haru countless times, but he seeks him out (why isn't all that clear to him in the beginning). He's scared at times and gets hurt badly, yet he is obsessed with Haru, and Haru is the same. He gets so irritated by Misaki that he resorts to violence, and takes his frustration out on him.
I don't think there could be enough of a "sad backstory" to justify the violence, but as long as they are both up for it, it's okay I guess(?). It's definitely not my type of "romance". It might have been intended to be one of those "this is terrible but I can't look away either" stories, but I've read a few that did it better, it just wasn't consistent enough for that. It wasn't really clear to me what exactly kept them together or how strong their attachment was, especially after volume 1 when the initial violent behaviour started to die down a little and they are attempting to have a more "normal" relationship. The main characters also seem to have a bit of an inexplicable personality change in the later volumes (especially Misaki). It seems the story wasn't really planned out that well to last three volumes.
I've read other manga by this author that I liked a lot more, apparently this was their first publication, and I think they improved a lot since then.
The translation was a bit odd at times, I feel it didn't always capture the right sentiment and/or got it wrong about who was speaking to/about whom, kind of like nobody checked for plausibility. Also - and this bothered me a lot - the official english edition of volume 1 didn't translate most of the sfx. To me it's fine to just have a tiny scribble beneath the original sound effect (if you don't want to touch the art), but to ignore it altogether is just lazy and takes away from the story.
(This review is about volume 1-3)