What I think about the bite is that was Keiichi taking out his frustration and hurt on Shingo. He didn't want to break up but let it happen anyway. He was shocked by the fact that Shingo could no longer smell him and therefore thought that if Shingo really doesn't trust him then he can no longer do anything about it.
On what he was thinking afterwards I'm thinking he was saying had he known things would end up in a break up he would have killed him so that he would always belong to him. But in the end he loves him to much to actually go through with it. That's what I got from it at least
i understand the same thing as you

I have a question about this bite: http://iweb6.mangapicgallery.com/imgfiles/kuroneko_kareshi_no_afurekata/7/030.png
Afterwards, Keiichi thinks, "Had I known earlier, I might as well just bite him to death and bring him over but I love him too much. I definitely won't be able to do it?"
When I first read it, I thought it was just post break up drama. You know, "I'd kill him if I didn't love him." Then I thought, maybe I missed something in the translation. It was kill him *and* bring him over: two different things. Over where? To some mystical big cat undeath? Would a big cat bite do something special to a little cat? And since they already broke up, what is it Keiichi definitely cannot do because he loves Shingo too much?
The picture looks like a classic act of submission. The little cat bares his throat to the big cat, and the big cat bites hard enough to show power. Then the big cat backs off and leaves the little cat. But these are magical cats who have human forms. Who knows what else is going on? This makes me wonder about all the things we don't know about this mythology.
Since we, like Shingo, are in the dark, my brain starts to churn. I wonder if Angelo did something that first night that gives him some kind of power over Keiichi? That would explain why Keichii lets Angelo dominate him and why Keiichi wants to keep Shingo and his brother away from Angelo. It may even have taken Keiichii's scent.
Of course, that's wild speculation. In my worst moments, I wonder if Angelo is a vampire and if he turned Keiichi into a vampire cat (or if being a vampire made him lose his cat). I read too many vampire books, but to me, being bitten to death and then "brought over" often means vampire. And if Angelo is now Keiichi's sire, usually Keichii would have to obey the master vampire. Looking again at the bite picture, if I didn't know the story, I might think vampire. But then again they go out in daylight.
Okay, that last paragraph was pretty far out there. But what you all know about the bite? Does it mean something? Could it? Any other ideas?