If you can't tell that this story is going to be on a toxic situation simply based on how the other male lead is a serial killer, I...simply don't have anything to say to you.
This comment is going to be full of spoilers, but with the story as bare as it is, might as well.
This is the story of how two people on the fringe of society, a killer and a high schooler who has been neglected and bullied all throughout his life, meets and gets into this weird relationship that neither could define.
The MC is a suicidal teenager, with parents that are at this point non-existent, and barely care about his well-being. Their only contribution to the MC's life is money sent regularly and his place of accomodation. Thanks to all that money, MC has been a constant target of bullying since bullies in the school can get away with it, given that he had long given up on retaliation and has not even one person standing on his side. The MC is used to people seeking him out purely for the sake of taking things he has (etc. the bullies seeking him out for the money), so much so that when the ML asked, he simply replied you can take it all away, my eyes, my tongue, my limbs and even my life, which shows how suicidal he is at this point.
The ML, well currently we only know he is a serial killer, but how he got to this point remains unclear, though it seems the author may explore it a little more in the future.
It's not a very deep story at all currently, but the main draw is in how both protagonist form a bond together and the ML inevitably begins to care for MC, who nobody actually showed genuine care for.
One area I noticed people have issues with is probably the two leads having sex, and their age. It is pretty obvious that this situation is unethical given that one of them is suicidal, but I think the MC does consent to this situation, and he genuinely does not care. The other issue is their age, the two leads have a massive age gap, 17 and 27. While technically that is the legal age of consent in Taiwan, obviously there will still be concerns of grooming and power dynamics. But currently other than them engaging in sex and living together, the ML doesn't seem to do anything coercive nor does ML try to build up the perception that their bond is special and try to use it against MC. Both of them clearly know their situation, but they seem to be in a weird limbo on where their relationship lies and have yet to confirm their relationship as even official, so its hard to say.