The existence of a hero would actually throw a wrench in his plans for peace, as it would allow people to perpetuate their differences and divisions under the pretense that the hero would take care of the demon lord.
Essentially, he needs to destroy the cycle of the existence of the hero that exists to destroy the demon lord and encourage humanity to unite independently of a hero so that their unity isn't a false, transient one, but one that will persist in fear of a future demon lord rising.
I think his plan will fall apart the second a hero arises, especially if the ones who control the hero use them to oppress those humans that have fallen under the demon lord's rule. I can see him eventually deciding to just unite humanity under himself rather than making him the object of their hatred/fear in that case.