Xin Yi was mu*dered on an ordinary night on their way back home from a class reunion meeting by his best friend, Ke Han, who seemingly had snapped under the influence of alcohol coupled with his long-term jealousy over Xin Yi's fortunate life and seemingly perfect relationship, impulsively committed an unredeemable act.
After this unhinged move, Ke Han fully derailed from morality and plotted to steal (NTR) the man whom he begot for the longest time by blackmail (first, the incriminating footage of the manslaughter of Sheng Fan killing his father and afterward, their forced s*xual intimacy).
The picture-perfect relationship of Ke Han was so envious of seems superficial at the end.
The man he loved, Sheng Fan, in the end, still valued himself the most and gave up on giving his deceased lover justice in order to protect his dignity, reputation, and future career. He shamelessly, though begrudgingly, spent his days next to Ke Han until a rebirthed Xin Yi exacted his revenge by seducing Sheng Fan who was reborn with a new identity as their new work colleague, Mo Li.

The first flashback showed the toxic relationship between Ke Han and Sheng Fan: Ke Han, after obtaining Sheng Fan, didn't seem to treasure him too much. He wouldn't give Sheng Fan much face, even in a public setting with work colleagues and people of higher business management.
Ke Han openly displayed his jealousy and controlling habits while Sheng Fan had already shown some shallow signs of cheating by defending Mo Li whom Ke Han belittled in secret.
Under this new identity as Mo Li, Xin Yi gained access to the truth of his death and betrayal of both lover and friend.
The indication of Sheng Fan in the shower while, Xin Yi (Mo Li) was using Sheng Fan's phone to call the entrapped Ke Han to mock him was an obvious sign.

Xin Yi made sure he returned the favor with interest and a poetic style.
Both traitors were left unspared, one tortured with mundane annoyances, horrors, and uncertainty til certain excruciating death of the same style as his own.
The other was left to face legal justice they've been escaping and avoiding, which was initially just manslaughter now was topped off with concealment of another homicide, all within mere hours on an ordinary night.

Though both were unworthy of this effort, I doubt it's something anybody could forgive if they were in Xin Yi's shoes. As inevitable as Xin Yi's unlawful act of revenge was, I hate that he had to be tainted by such sins.
Morally speaking, there are no good guys here.
To vanquish evil, one must become a greater evil.