My impression was that it was about someone that really really hurt him, hence the "you are the thing that's killing me" part, because it feels like that when someone has broken your heart or something similar. And perhaps they are the kinda person that does it on a regular basis, hence the "you're a monster, you're built to fall" part, saying that, because they do this a lot they will fall inevitably, because they are a horrible person. Just my interpretation though.
My impression was that it was about someone that really really hurt him, hence the "you are the thing that's killing me" part, because it feels like that when someone has broken your heart or something similar. And perhaps they are the kinda person that does it on a regular basis, hence the "you're a monster, you're built to fall" part, saying that, because they do this a lot they will fall inevitably, because they are a horrible person. Just my interpretation though.