It's Not Easy to Be a Man After Travelling to the Future (WN) - Chapter 1309: Assumptions.

When we lose our hearts to the wrong person, setting a betrayal in place, how well do we come out from such hurt?

Rocco Di Malvento lived a short, but loving life in his previous world, surrounded by a caring family. Despite suffering from a terminal illness, he found solace in books, especially his favorite BL novel >. But when he dies, he wakes up in an unexpected new life—as the weak, neglected second son of a powerful mafia family.Now reborn in the world of the novel, Rocco finds himself in a terrifying situation: he's the useless younger brother of Sylas, the main protagonist, and likely doomed to die young unless he changes the course of his life. So, what's his plan? Simple—befriend the protagonist, avoid being a spoiled brat, and secretly recruit Sylas's future allies to protect him.First target? Georgio the Hyena, an infamous killer. Rocco's strategy: charm the terrifying man and make him his ally instead of Sylas’s.It's a ridiculous plan, but hey, at least Rocco is trying. Can he survive the mafia world and avoid his tragic fate? He hopes so. After all, he’s just a kid trying to make the most of his second chance!

“The lifespan of a butterfly is between three days and a month, and most butterflies only live for a week.”
“Like them, I have only a few months to live, maybe less.”

–

The first time Hu Die met Jing Yu, he thought she was a world-weary girl who committed suicide.
The second time they met, he told Hu Die indifferently: “If you jump down again today, I’m not saving you again.”
Later, she told him: “Jing Yu, don’t feel sorry for me.”

Later, Jing Yu returned to the arena and won the glory that belonged to him.
Under the flashing lights, he suddenly remembered the girl’s voice, and said a nonsensical remark in front of thousands of people.

——”I’m not sad, I just think that at such a moment, it would be better if you’re here.”

“He is a whale swimming in the ocean. One day, a butterfly inadvertently broke into his frequency. That was the most beautiful moment in his life.”

- Description from Novelupdates

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates