Demon of the Baron's Family.

When an ordinary high school graduate wakes up inside his favorite webnovel as Osborne von Bismarck, the second son of the prestigious Bismarck family, he decides on one thing. To stay far away from the story's main events. Armed with knowledge of the original plot, surviving should have been easy. Except the moment he begins acting differently from the original Osborne, the story starts to change. One event becomes two. Minor characters start making unexpected choices and the future he memorized slowly begins to slip out of his grasp. At first, he doesn't notice the changes. But when the wife who was supposed to despise him starts becoming frighteningly affectionate, and increasingly possessive, he realizes he's gone terribly off-script. Now, trapped in a story that no longer follows its own script, Osborne must figure out what he changed before his obsessive wife refuses to let him go. — Description from novelcodex.org