I think this song is obviously about a serial killer trying to kill someone who is a cancer survivor. Read the lyrics, and it makes sense, she is a master of metaphor.
In the first part, she says that she cannot step towards him (because he's about to kill her) and she says that she learned to live half alive with her cancer, but won't let him take what she has left.
In the chorus, she is wondering who the killer is, because he is likely wearing a mask. Obviously he has a jar of hearts (trophies) from each of his victims until this point, and many times he has killed someone who was part of a couple, and thus he "tore love apart." He is also probably sick because he might have to live outside or hide a lot, and thus has ice "in his soul" (another metaphor) which gave him the flu. She says don't come back for her, because he probably killed the group of friends she was with, but she doesn't want him to come back for her.
Then you can see, that the killer realized that he didn't get her, and she is a witness, so he is trying desperately to find her. However, she is recovering from the cancer, so she's now "too strong" for him to catch her, he's likely not very strong, he just usually gets the jump on his victims.
Afterwards, it's revealed that she might have had an emotional relationship with the killer. She found the light inside her eyes, but only after it was reflected off of his jar of hearts that she found in his closet which is why he was trying to get her in the first place. Now he wants her back, but if he can't have her alive, at least he'll always have her heart.
Now at the last part, she is just questioning him. Wondering if he even knows who he is anymore after killing so many and keeping their heart in a jar. It's really a very haunting song, it makes you wonder how someone could do something like that.