With every new chapter, Yakushimaru-chan is getting less and less likeable. Ootaki-kun did nothing wrong or inappropriate (of his own volition), and especially nothing that would give her the right to slap him. Granted, if she was actually his girlfriend, I'd understand, and perhaps even approve. But they are friends at best and, especially since she is the one who friend-zoned him, Yakushimaru-chan has no right to bitch and moan about his behavior, even if she had caught them kissing. Yes, I think the word "bitch" seems appropriate here.