Gachikoi Maou-sama

If I destroy the human race - then will you stay with me? A hero took on the task of defeating the Demon Lord for the sake of world peace. When he arrived at the Demon Lord's castle, he was greeted by the Demon Lord herself with a marriage certificate ready to go! The hero was puzzled by the Demon King's strong desire for him. A love comedy about the world's survival between a stubbornly in-love Demon Lord and a tsundere hero! (Source: Kadokawa, translated)