Demon Lord, He Regrets It

I used to be the most carefree little junior sister in our school, until I picked up that dying boy. Master figured out that he was the reincarnation of the ancient demon god and advised me to kill him to avoid future troubles. But when he opened his eyes and looked at me ignorantly, I just hid him softly. For hundreds of years, I helped him regain his position as Demon King, but he led thousands of demons to destroy my master. The moment the sword pointed at my chest, he chuckled: "Senior sister, you said... You will always protect me." As I crushed my natal sword, I thought: If I had known that it would be so disturbing to people's hearts, why would I not have known each other in the first place? ... I used to be the ancient demon god in charge of life and death, until I was seriously injured and died, and she took me back to my master on a rainy night. She was the only fire that threw herself at me regardless of anything during my long years of loneliness. My master advised her to kill me, and my classmates forced her to abandon me, but she hid me behind her back and said she would always protect me. After a hundred years of companionship, I absorbed her warmth and ascended to the position of Demon Lord again, leading thousands of demons to conquer the Immortal Gate. When my sword was pointed at her heart, I was shocked to realize that in the chess game that had been calculated for hundreds of years, I had already turned myself into her puppet. When she crushed her natal sword, I finally understood: If I had known that it would be so disturbing to people's hearts, why would I not have known each other in the first place?