Spoilers:
1. His friend took suicide because he was gang banged by the ML's friends. abuse and used him for the ML's revenge plan.
2. ML approach the MC cuz he thought MC knew about his secrets, thinks that the MC's dead friend told the MC about his revenge plans.
3. ML's dead fiance took suicide because she cant do it anymore, she was just used and blackmailed by the ML's revenge plans.
4. ML's dead fiance's younger sister and the ML were old fuck buds (ML also used the younger sister as a tool for revenge) (this happened before they met with the MC)
5. As the time goes by, the ML gones quite possessive and crazy for the MC. falls in love but keeps all his secrets away.
6. Taeshin sent MC a letter (in box with diary and other belongings) of all the cruel things the ML did to him but MC did not even care to look at it and throw it away, which ultimately got in the hands of ML who burned it. I get that you never felt close with taeshin but after he killed himself how could you just toss his belongings and not read a letter he sent to you.
The ML has this secret villa that he invites chaebol sons or rich people to party and do some illegal things like orgy, drugs etc without them knowing they were being filmed and blackmailed later on. He turns them into his pawns for his revenge. When he was dating and manipulating Taesin (MC's dead friend) he invited him on his private villa just to hangout , but people there assume that Taesin is the new playtoy for the villa. MC leave for a bit and left Taesin alone, there six people abusely gang banged Taesin, this caused trauma, he eventually escaped but lost in the woods for 3 days. Taesin is now mentally disabled and decided to suicide due to betrayal, abuse and blackmail. just like what happened the the ML's dead fiance, both of them are 29 yrs old... wc is also the same age as the MC right now. its like a jinx or something.
Into The Thrill