Akugyaku Kizoku no Shinpen Seiri

After careful consideration and meticulous planning, Kashimiya Tsukasa ends his life to save his family. However, instead of reaching the afterlife, his eyes snap open to scenes of torture, mutilation, and so much pain...all inflicted by his own hands. When his head clears, he realizes that he's been reincarnated into the world of his favorite novel—not as a hero, but as the sadistic Duke Heinreid von Rodvelia. Haunted by the memories of the duke's vile deeds, he vows to atone by giving himself the permanent ending he deserves. But soon, his shift in personality arouses the suspicions of Craig Bauer, Heinreid's personal knight and the one who finally kills him in the original novel. Craig's attention slips into obsession, and as his fate intertwines with Heinreid's doomed destiny, the story of the novel begins to warp around them. Will Heinreid end up skewered down to the hilt of Craig's sword? Or can even the most despicable duke find a reason to keep living beyond the final page?

(Source: J-Novel Club)