I Became the Villainess in a Disastrous Novel

I became the villainess of the novel I was reading, but that novel...had just about the most anticlimactic ending imaginable. After a long and slow romance development, the empire was suddenly destroyed by a demon on the wedding day of the male and female lead! To hell with the original story! First of all, I have to try and survive, so I want to flee to the neighboring country, but then... "I've waited for 100 years just to meet you." The man who had hidden his identity as the Demon King and was the driving force behind the destruction of the empire, Grand Duke Arvis, grabbed me, his gaze filled with intense emotions. "If I lose you... I'd probably start by wiping out this continent that took you away from me." ...Just why? (Source: Naver Corporation, translated)