Some flaws...

[Kedjane]

What kills you in the dark is indeed relatively surely Frisk. Either a dog, small rock worm or Frisk but since we know Frisk killed Spider Hater (Roberts, yes) we know he's also capable of fitting through the small tunnels making it more likely it was him. And he hangs around the kennel, too.
I haven't noticed the note being named "monster note", but I don't think Frisk is related to the hunter since it was cut already in the tech demo and Frisk wasn't in the tech demo.