In the original, Yeonsoo is jealous bc he cant keep the ML attention and their compatibility is lowering, meanwhile with jun is rising and ML becomes obsessed with jun. Yeonsoo also become obsess to manifest as an omega to get the ML to look/love him.
He dies by ML/drug. I think. at the same time, he took a drug to turn omega but couldn't overcome the side effect. The end on that.
Now this timeline is a little complex, where it begins, there's an important part in the past that makes the beginning of the story start there and why ML "become" obsessed with yeonsoo
(The webtoon, is going to fast so you might reach it soon)
Ultimately, yeonsoo does something different when he remembers he's in the novel and thats when the ML focuses on Yeonsoo.
Its a really nice novel! Give it a read! The webtoon imo isn't doing its justice!!

Webtoon pace sucks! Its going too fast and the eyes on Ml art style is very shoujo vibe lol I feel like the art style changed..
All I have to say is I enjoyed reading the novel lol