Idk man Xin's my fav character. I think a point of the story is that everyone is flawed so I can get why you think everyone's an ass, but IMO by that same logic Lau should be considered an ass too. Lau has also made decisions that hurt other people (and my heart TwT) even if they weren't warranted because of his trauma. IMO Xin is the same way, it's just that we don't get to see his thought process as much as Lau's so we don't fully understand why he does certain things.
I think the purpose of this current childhood flashback arc is to show why Xin & Lau are who they are in the present. In a sense, I think this arc is supposed to be the sob story you're referring to. And it does answer some of the questions like why Xin treated Lau that way in the present. He's emotionally stunted due to being shut off from the outside world for his entire life (plus he was lowkey raised in a cult), and as a result acts very creepy and bottles his emotions inside. Lau has abandonment issues from his sister leaving him which can cause him to lash out at people. In the storybook chapter, we can see why Xin burned their contract in the present: to save Lau from the head family.
The next few chapters are really hard on both Xin and Lau so hopefully you'll be able to sympathize with Xin more. Despite him being creepy and stuff, I do think he's a good and well-meaning kid. And I think he's still well-meaning in the present, even if he doesn't make the best actions. But I also think that he thinks those actions are the best he can do to save Lau, even if it means sacrificing himself in the process. We have no idea what the head family can do to Lau, but Xin probably does, which is why he is taking every measure he can to save Lau from them. Hopefully, he grows on you. May I ask what specifically makes you not like him?
Yeahhh I think so too. Its probably a combination of that + him not wanting the head family to get Lau, so he severed their connection. I agree that its selfish but I think its been well established that in Xin's mind, Lau's safety is worth more than salvaging whatever relationship they had left. That's why he specifically taunts Lau before burning their contract (maybe taunts isn't the right word?). I think Xin's trying to sacrifice himself for Lau, like in the storybook. So yeah its still pretty selfish but ig he has his reasons.
i kinda dont like xin, gimme his sob story quick or imma start wishing for lau to find his happiness elsewhere