Isekai - abandoning humanity to defend the half-beast race

Kira was one of 20 students transmigrated to a generic isekai, with the aim of defeating the demon king, after 30 years of struggle and suffering, the demons finally succumbed and were expelled from the great continent, but all Kira gained in the end was the ingratitude of humanity and betrayal of colleagues, Kira then decided to abandon his position as a human hero and moved to the small and weak country of the half-beast race, the only ones who were not ungrateful.
Who could have imagined that just a short time later, the demons returned but this time with more than 100 demon lords?
Finally Kira discovered that the continent he spent 3 decades fighting on was nothing more than a damn "newbie village", fortunately thanks to his efforts he was rewarded with a system