Hmmm, with the text surrounding hikaru saying "leave her to die" im assuming she had some part in Hina's original disappearance.
Maybe, Hina slipped and hit her head while playing with Hikaru in an area they weren't supposed to be, and instead of rushing to get help Hikaru didn't want to take responsibility or get in trouble, so she left Hina there.
Maybe its something less direct, or maybe her guilt is simply from not keeping hold of her like she mentioned in the previous chapter. Certainly doesn't seem like she was that involved in her disappearance like in my former theory from how she's been acting up to this point.
The text also says "Can't be Hina" I wonder what else she knows about Hina's original disappearance that makes her think this.