I think Clare has done her due dilligence in warning Raki about her lifestyle, and has given him every opportunity to pursue his own safety. But he and she both realize that the pursuit of happiness supersedes the pursuit of safety.

I think it's possible that Clare's thinking is becoming less claymore-like and more human-like since her half-awakening. As in, before she wasn't really capable of having romantic feelings, or at least wasn't capable of letting herself do so. But post-awakening, I think something's changed about her mindset a bit. Consider the implications of "awakening" as a bit broader than just "turning into a freaky tentacle-demon"...

I'm pretty sure Raki realized that Clare was more than a little overmatched facing Ophelia. That's precisely why he wanted to stay with her: in his mind he was faced with the prospect of either letting her go alone and losing her forever, or staying with her and dying with her. The function of the kiss and the promise to return is an assurance that she will not die, that she has something to come back for and has every intention of coming back, and is not going to throw her life away so easily as she might have before.