The unfortunate ending, Rin route?

[ScrambledEgg]

I was bewildered by what appeared to be Riki's sudden death? After ducking out of the classroom, Rin started explaining how to do a proper tiptoe in the corridor. Suddenly, Riki seemed to have a seizure and everything went dark. Several texts appeared "congratulation you make it this far", " nothing has started yet","Things will not go so easily" and "The paths you didn't choose, try to accept it next time". At the end, Riki mumbled that he was terrified and then the title appeared.

This is my first route, but everything felt kind of off after Rin had confessed her love to Riki. I was no long saw any female characters from before. We did the mission that Lenon brought as usual and this time Rin and Riki were assigned to be a guide for school inspectors. Everything went well and Rin was chosen to be a representative student to help another school recover their morale after the traffic accident that only 3 students survived. I refused to let her go and she complied. We spent our days as usual but I didn't see other friends anymore. I got a bad feeling and then this happened.

What happened to Riki? And what the hell did I do to get this result?
I wasn't even sad when I saw the ending. It was more of disturbing

[Icku]

Just finish the game. Answering would be a spoiler.

[caetharus]

You need to clear the other routes to learn the secret of the world.