Not quite sure how to fully interpret this, but here is my understanding:
The kid actually went to an abandoned hospital, where there is some severed head lying around.
He said he visited with Miyo, but on the 1st and 4th panel, you can noticed that Miyo does not have legs, and he had drawn legs for himself on 1st panel, so it could not be a mistake.
I think he has been seeing a ghost of Miyo, and Miyo is actually murdered in the hospital. Her head might be one of the severed head the kid saw.
He also mentioned "This is where her head was cut off mistake.". The past tense "was" might suggest that he was at the scene when Miyo was killed. No idea if someone have killed Miyo by accident, or the kid himself accidentally killed her.