
I don't think One is a SM dominant yet ! Right now, he is just a human slave who hate this vampire society. If He start to love is partenaire things should change I hope !

While I agree 100% that this is not a healthy relationship, One does't view it as a relationship. This is an activity that endangers his life. He is 1/2 a step away from being a POW. Where vampire pheromones help keep the human populous placid, they don't effect him. So when you give a prisoner with a hella lot of repressed rage free reign to get some of there own back it isn't going to be pretty. One from the beginning has asked to be left alone.

Something to keep in mind is that one wasn't into and never/doesn't really understand BDSM play. He just kinda tried to follow what Shell wanted.
I don't think One is a good dominant because he loses control easily and he lets out his anger on Shell. He hurts him not because it's what Shell wants, but because it is a convenient way for him to vent out his anger. That's not what s&m relationships are about at all. He doesn't have any limits, they don't even have a safe word, but i'm not sure One would respect Shell's limits anyway, and would end up hurting him when he is angry.
I like the bdsm aspect of this manga, but after realizing that the only reason One is doing this is because he is angry and is using Shell to vent out his hate towards vampires, i'm only seeing abuse. It's true that Shell asked for it, but there is a difference between hurting someone because it gives them pleasure and hurting someone to vent out your anger. Same goes for Shell, he wants someone to dominate him, that doesn't mean he wants someone to take their frustrations out on him.
That's really not s&m, it just comes down to abuse.
Since the main focus is bdsm, i think hopefully it will be portrayed properly at some point.
this is up to chapter 16.