For Kaori even more than for Inori and Motoko, it was far from coming out of nowhere. This kind of build-up is what makes a big difference compared to stupid, purely pornographic games with zero context, zero build-up, and so on. You can't decently switch girls like Motoko and Inori from innocent and shy young girls to uninhibited, promiscuous sluts; it wouldn't make any sense. The MC wants revenge on those responsible for his father's fate and doesn't hesitate to sweep aside those who get in his way, but he's not a complete psycho either. Building up the tension gradually, without really being sure of capturing the heart of the 'belle", is part of what makes this game appealing. What's more, offering Motoko and perhaps Inori (although much less likely for the young redhead) as a kind of redemption gives meaning to the Karma system.