I think you are right--it is about control. The question in my mind is whether there are any feelings for Azusa (now or ever) or if Azusa is realy just a toy.
I think there are feelings but Karino doesn't even understand them,like when he kissed azusa in chap3 and didn't know why he did it. I really hate it when azusa is treated like a toy who will be thrown away after graduation >.< chap4 when azusa went to karino's home was painful because of this(actually for me chap4 was more painful than chap8 simply because it revealed the fact that azusa could be simply a toy and azusa himself started wanting to be with Karino...it was so sad to read ç.ç)
Sometimes I think like that about Karino. Sometimes I worry that Karino is a psychopath and the point of the story is that feelings make you weak and those without them will always exploit those who show such weakness. Without knowing more about Karino, I just don't know.
There's no such thing as Karino protecting Azusa,he wants to control Azusa. That's what he wanted since the beginning,he was feeling bored at school and azusa attracted his attention,considering that karino seems a sadist,he clearly wanted to have complete power over azusa. His possessiveness is much more psychological rather than physical,he can't stand when azusa finds support from someone else (such as kusakabe) because he won't have total control over him. It's not about who fu**s azusa,but who owns azusa.