You decide to drop a bit of Holy water into the ocean yourself. Given that the church wasn't going to do it for you. You manage to aquire some holy water, you asked a local priest to help contain the supernatural, but you did not tell them what you planned to do. The priest didn't question it, since you're a trusted member of the church, but you did not tell him what you really planned to do with the holy water. You've been told not to do this when you first joined the church, but not why, most priests seemed to understand why even without the reason being specified, you didn't and you faked obedience until you were given exactly what you wanted.
At first nothing happens. Than all over the world it happens.
The supernatural creatures start dying. But not only the bad ones. As it turns out there are millions of perfectly good demons, spirits, undead and other supernatural creatures, most of them living as humans, die suddenly. Millions of citizens all across the world die, hiting larger numbers of casualties than any other disaster. This naturally not only crashes the world economy but nations suddenly lose multiple experts, doctors, engineers, diplomats, generals, every conceivable and inconceivable role in society.
But that's not all, several people are half supernatural, some of them don't even know it, their powers manifesting in minor ways. Since they are not fully supernatural, they don't die due to the holy water. But they're now severly disabled. Now most of them require a caretaker to survive, and a lot of caretakers knowledgable on supernatural and half supernatural beings, have just died.
We're still not done, however, there are several supernatural ecosystems, Haunted Woods, Haunted Oceans, Magic Deserts, and many more, that not only do supernatural creatures rely on, but some non supernatural creatures as well. Leading to not only the extinction of all supernatural beings but many, many non supernatural species as well, making this the largest ecological disaster in history.
The cherry on top. Without a common enemy in the form of demons, holy beings such as angels have no reason to be allies of humanity anymore. They've taken the place of demons, stealing, taking advantage of, killing and many other hostile actions against humans now that they don't need them as allies anymore.
Good job, you've destroyed the world. You may claim this is not your fault, rthat the church didn't warn you of the destruction that this would bring. But this is all your fault, you knew, hoped that all supernatural creatures would die when you did this. And now they have. You wanted genocide, you got genocide. The only thing you didn't know, is how badly it would effect you. The only shame is that no one knows that this is all your fault.
