Interesting idea, though you can find lurid stories of all too human mass murderers that pre-date Jack the Ripper by quite a bit. Vlad the Impaler and Elizabeth Bathory come immediately to mind, as does the likely fictional but often touted as true story of Sawney Bean
But then, they were famous, and their deeds could hardly go unnoticed.
I'm afraid that there's a percentage of the human population that is serial killer material, and that this number might have been steady throughout the years. It's human nature. Fortunately, it's a minuscule percentage.
But what about the villager who is a sexual predator? Upon seeing the horror of a mangled body lying on a ditch, who's to think it was their own neighbor who did it? And then... how many deaths that were then attributed to "evil" powers were indeed the work of human beings? In this regard, monsters become a very suitable scapegoat.
That's not the whole story - but part of it.
Still, it's an interesting idea, and you may be on to something there. Monsters have always fascinated me, even the human ones. This thread may get me off my butt and start doing some reasearch into why people invent monsters. The more I think about it, the more interesting it sounds.
Do it, and don't forget to mention Marco Polo and the Spanish and Portuguese conquerors, who had to fight many sea dragons and giant squids to reach the New World.