The thing is, we don't exactly know the reason for why "Cataclysm" killed humans and assuming the reason was "just for fun" doesn't seem like the most reasonable one. There is a theory about how Cataclysm was a victim and result of experimentation done by humans and several people have turned their attention towards it. To be honest, it would be a lot more cliché and boring if Cataclysm did end up as the "true villain" all along. It wouldn't be very shocking and we would most likely already have expected it seeing how in the first chapter Charlotte killed him. Whereas if it was an ally and one that we wouldn't expect it from... that would be something. We might as well as expect a big plot twist at this point.