I'm gonna throw out a long shot... these demons are fairly well-versed when it comes to humanity. What if their intent is some kind of long-con? Offer to surrender peacefully, Rem obviously doesn't take up the offer and retaliates violently.
Then the demons go public with their sapience and claim they want peace, while showing off how the Anti-Demon Corps viciously retaliated in response to an attempt at diplomatic resolution. Combined with public revelations of the Anti-Demon Corps' very real mistreatment of innocent victims and hybrids, and suddenly public perception takes a nosedive.