Aku Ochi Labyrinth ~Toraware Maou to Naraku no Kyoujin~

Claude of the special Academy of Nobles wasn't skilled in battle, magic or wits, and was insulted for his shortcomings by other students. In his heart he yearned for chaos to engulf the Academy.

Claude encountered Shiesta, the timid demon queen of the Hellrealm. She was compassionate, hoping that demons and humans could share the world. She had come on a mission of peace. But he only saw the chance to use her dark power against his fellow humans. Chaos would engulf the Academy, and he would be its master.

Claude lured Shiesta to a cave outside the city and exploited her natural kindness so that he may rule with violence and terror. He summoned the monsters of Hell in her name and transformed the cave into a catacomb dungeon. Then he made war with the humans.

[Edited from DLsite]