@Starbright
As I mention, I don't think Demon king is able to reincarnate due to the vague legend story saying that he was buried. He could be sealed and Airi is summoned to become the new Hero(Messiah) to deal with the loosening seal of the Demon king that is about to break. The novel that Airi was talking about on page 1 is about
Heroine character not the Hero related to the Maydare legend. Heroine as in
Female Main Character. On page 3 she lends Kazuha/Maika a manga related to fantasy and own couple more of them, meaning that this is more of her personal interest on the genre rather than vague memory of the past. It's probably just a coincidence/trap on the part she said "heroine smites this super evil witch" on page 1. I think the hair color thing is also a trap too. I mean there is probably a magic to change hair color, appearance or a hair dye lol.
The blonde probably suicide after they murdered 3 of them and follow them into the wheel of reincarnation. Just like I said previously in my speculation, their fate would intertwine with each other. On the page 16 the blonde say Maydare after the murder meaning that "I will met you all on there". So it's a guarantee that they will reincarnate in the same Maydare world.
For the confession thinggy, I think Airi is trying to stir up the situation and trying to get a reference material for her novel. I mean her action doesn't really look like a supportive move to me. On page 7 it shows that Airi tends to do same things on Kazuha/Maika. She probably already started using Kazuha as her character reference long time ago for her novel and trying to get in the same shoe of the character. To get the feeling of it.
It could be that blonde White sage is the one that created this whole mess of reincarnation by using reincarnation magic that he created. So this is probably the only reason why he remembers all the past but the others only piece and bits.
@renuac
If blonde main purpose killing all three of them that doesn't adds up by the order that they were killed. On page 9 blonde already went pass Kazuha/Maika once and he should be able to kill her first. He start killing her after she enter the rooftop. The reason why he kill them all is because he wanted to put them into the wheel of reincarnation. On page 16 he said "Maydare" is the solid proof of that, because the word Maydare in Earth doesn't mean any shit. What he is probably trying to say is "I'll be meeting you all in Maydare". I did write my speculation on the blonde's identity and his purpose on the comments, try check it out.