Super basic summary but Haebom (MC) is adopted by Taesung's mom after his parents die in a car accident. Haebom thinks Taesung (ML) doesn't like him because a long time ago Taesung said he didn't want Haebom as his brother.
A lot of sweet things happen in between them and they eventually fix the misunderstanding between them. Taesung didn't want him as a brother because he wanted to marry Haebom. They end up dating, but their mom doesn't approve because she worries they will be ostracized by society. Taesung and Haebom are basically like we don't gaf we love each other and she eventually gives them her blessing.
In the following seasons we basically see their life through college and becoming working adults (Lots of fluff). Each season they meet a new bad guy and eventually resolve it.
In the latest season, we take a pretty dark turn. Taesung's maternal grandfather blackmails Haebom into leaving Taesung forever so Taesung can marry a girl, blah blah inherit the company. Haebom is kidnapped and abused, and Taesung basically loses the will to live and keeps trying to find him. Taesung eventually finds Haebom working at some random bakery and he begs him to come back but Haebom refuses (because he's been blackmailed). The next chapters are basically they have sex, and Haebom never tells Taesung what happened despite him asking. Haebom eventually gets the courage to tell Taesung but the grandfather kidnaps him again and tells Taesung to leave Haebom forever or else he'll get sexually assaulted by some red head dude he hired.
Taesung agrees and we get the typical "I'll listen to what he says for now but I'm planning my revenge". Haebom moves in with their friend (i forgot his name but the cutie with the blonde hair) and gets a lot better while Taesung gets a lot more emo. Taesung goes on a blind date with another rich ceo's daughter and they plan to take down his grandfather by exposing him for kidnapping Haebom. A lot of stuff happens but eventually they corner the grandfather into giving Taesung all his company shares so the marriage can go through. Taesung gives the blind date girl's family all of the shares instead and runs away from the marriage.
Taesung is finally free and gives Haebom a ton of money along with a letter that says please be happy without me. Chapter ends with the reader thinking Taesung is about to jump from a 50 story building LOL. He doesn't jump and he reunites with Haebom and they have make up sex. The end

WARNING: SPOILER!!!
Will someone be kind enough to summarise this story for me? I don't even remember upto where I read, but back then it wasn't angsty. Now I just had a look and everyone is talking about it going downhill. What exactly happened in the story?