Oscarus

And then he and his wife got killed because of the keys, causing their son to create an organization and went off into the madness

That's just sad and cruel, which makes me feel bad for Solaris

That makes me wonder... If Taube's and his wife's death could be put as "the chronologically first causation of Pokemon Reborn events" 

Because of that, Solaris began his path toward evil (which isn't as rare of a trope as one might think), and decided to protect the keys himself, as previously he hadn't cared previously about them or family's traditions. 

But... that didn't work out 'cause Radomus yoinked them out of his sight (except Emerald Brooch which had been given earlier to Elias), only to sell Ruby and Sapphire to Corey and Monthy, which were given to their loves - Elena and Caroline, and gave Amethyst to Zina, alongside buying castle south of Route 1, as apparently she dreamt of living in castle. 

Soon after, he formed Team Meteor to fulfill his goal - reshape Reborn City to its original "form". And then... we all know what has happened