
Called it!
Based on the raws, the lil bro should've been the one who dies. There's this number/year timer that showed near our lil bro's head before our taehyuk got killed. Who's the killer? The big bro.
On Chapter 18, while our pair is at a park, the big bro comes with someone. Maybe a friend to give some thoughts about the pair's relationship. But damn, our Taehyuk will get hit by a wood in the head as an ending scene. That weapon came out of nowhere, it's hilarious (≧∀≦)

I could be wrong but if you read the raws, you'll think the same as me. Why would our Taehyuk see his own death timer? It wouldn't make sense if he did coz why was it placed at his perspective of Hangyeol?
But to answer your question, we see that big bro wants lil bro to be in his grasp at all times. May it be physical, emotional or psychological. He commanded Hangyeol to smile despite being blamed why he went home early, didn't he? Hangyeol getting out off his grasp by liking a man and showing more genuine emotions could've been the reason why his anger towards Taehyuk got that immense thus killing him via car collission.
Now, to match my understanding of the raws and my reasoning on big bro's behavior/obsession, it could be possible that Hangyeol would've died if:
1. Lil bro pushed Taehyuk aside first;
2. Or big bro's speed caused the tire to sway (it was winter, wasn't it?) thus hitting lil bro.
Still, one thing's clear. The big bro is the reason why the shit happened (╯°Д °)╯╧╧

I thought the same too ... I haven't read the raws yet, but I had a hunch. The way big bro behaved with his little brother...was obsessive and dominating. Let's not forget how the little brother was intimidated as a child and never expressed his own opinions. His brother always oppressed him in the name of love and care.
So I had a hunch that maybe the accident...can be caused by the brother. Big bro wouldn't let his little brother out of his grasp.
This is fiction, take note of it. Yes, I agree with everything here, BECAUSE I'd rather have my mc be treated like this instead what he's been dealing with before he met the ML. It might have tweaky morals but isnt it living more important than any other setting? If the ml makes the mc live, i'm okay w the things he does to him. Besides, there's no physical abuse. We see him taking our mc to buy new clothes, eat new food/drinks, play w/him. Id rather have this than any other scenario ╥﹏╥