It's the first one:I seem to recall having seen a flashback of Korosensei in the process of becoming the purple-thing he is nowadays while holding a wounded/dying woman in his arms, with her being the one to tell him to teach kids as he'd make a wonderful teacher. I'm going to guess Kayano is this woman's sister, although I have no clue as to in what chapter did that scene appear.
Pretty interesting chapter in any case
Very likely that this woman is Yukimura, so the clash between Kayano and Korosensei is bound to reveal some stuff about the past. I like Hamy's theory too, it doesn't contradict that Yukimura was a former student of the chairman. Definitely looking forward to how this will turn out, the last two chapters have been excellent

Kayano's backstory makes a lot of sense, and you can't really blame her for thinking Korosensei killed her sister. and her assasination attempt's preparations were indeed thourough, considering how she arranged the ENTIRE thing up to her attempt.

Nagisa is going to be a great assassin one day, combining Irina's seductive tactics with his own elements of surprise.
That's why I really like Matsui, you never know what to expect and then he comes up with something as bold and amazing as this, kudos. Surely Kayano and Nagisa could be one great assassin pairing 