
Goddammit I didn't know this was a spin-off, did I saw spoilers from the main series? Is it worth reading the main series even though I read the spin-off first?

You reading this spin-off won't affect your reading of the main series that much. Aside from the pd, no other character appears there. It's about a different couple and the pd might be a "2nd ml" but I think his presence wasn't really equal to the 1st ml.
Also the spin-off is mix of past and present, so the main series is actually in between. So the only thing you've basically spoiled to yourself about the main series is who's the end game.
I personally thing the two webtoons give different vibe. This one is more mature and serious. While the other is a bit cutesy and funny.

Does anyone know how the brother died?

I said that bc he is a freak who wants all attention on him, I can't imagined he being ok having a twin. And well there's some of criminal cases that have be a similar situation like a twin or a brother killing the other bc they can't stand the other and have to be the only one, and some of these where done in quite young age, it's was disturbing young age. But I hope we learn more about the twin death. And I kinda wanna see the parents discovering the all the truth, in case they didn't know, I wanna they suffer for not looking after ml and let all this happen. I'm not found of ML family after learn this past and I like ml more too bc him turn kind well/good(not sure what word I should use) considering all the shit he has gone through from a very young age.
If you say I was good go to hell I was be scared, but if you said that this asshole was gonna be there next to me I will be scared to death. He doesn't have a screw lose bc all his systems are spreads all over the place, doesn't exist a single screw on that brain of his.
ANYONE KNOWD?