Kujibiki Tokushou: Musou Hāremu ken - Chapter 251

A story in which a woman, after learning that the world would end in ten days, sets out to finish one last thing: murder her ex-girlfriend.

However, she ends up befriending, and spending the world’s final days with, the little sister of her ex-girlfriend instead.

What happens when a scientist from a futuristic world reincarnates in a World of Magic and Knights? An awesome MC is what happens! A scientist’s goal is to explore the secrets of the universe, and this is exactly what Leylin sets out to do when he is reincarnated. Dark, cold and calculating, he makes use of all his resources as he sets off on his adventures to meet his goal. Face? Who needs that… Hmmm… that guy seems too powerful for me to take on now… I better keep a low profile for now. You want me to help you? Sure… but what benefit can I get out of it? Nothing? Bye. Hmmm… that guy looks like he might cause me problems in the future. Should I let him off for now and let him grow into someone that can threaten me….. Nahhh. *kill*-

Alternative: Leylin is transported from an advanced technological era into a medieval world— except there’s magic. Equipped with “only” his quick wits and an A.I. chip fused to his soul, he sets out to become the most powerful magus the world has ever known. In a land divided into the Light and Dark Magi, the weak can either be cattle or disposable slaves. Only the strong can determine their lives, and only they have the luxury for virtue. Yet strength is only a tool for Leylin, like any other. His goal, in the end, is his own.

The genius cultivator Lin Xuanzhi didn’t let down the world in his past life, yet he only betrayed a single Yan Tianhen.

It was only when he had been backstabbed and killed by his closest friends, teacher and fellow disciples that he knew what kind of unforgivable crimes he had committed.

After he gained the huge opportunity for rebirth, the Lin Xuanzhi who had climbed back up from hell swore to cherish the person that he had betrayed in his past life.

- Description from novelupdates

He died on the wedding of the girl he had a crush on, after a long and successful career. After convincing Yama to send him back to the world, he decides to fix the areas in which his life went wrong. Realizing the lack of love and romance in his past life, he sets out to conquer the hearts of various women.