The good end reveals that the monster has been Miki's aunt this whole time. After Miki asks Saeki's forgiveness, you see the scary images you usually see when the monster catches and kills you in a game over, but this time it comes across more like a hug than anything. Has Saeki just been trying to hug her niece and tell her everything's okay this whole time, but in all those game overs kept accidentally killing her instead?
In the bad ending, Miki is found dead, having apparently been killed by the monster. However, the good ending implies that the monster, who was actually her aunt Saeki, was possibly a hallucination brought about by Miki's schizophrenia. So how did Miki die? Well, Miki had obviously been suffering from depression for some time and moments before the monster kills her, she says she blames herself for Saeki's death. Therefore, is it not plausible that Miki killed herself, unable to live with the guilt any longer?
In one of the puzzles, the player has to decipher a note which reads, "Silence, happiness, peace. I see them all in a row, but it's happiness I can't have." To solve the puzzle, the player has to go to the dress room and look into the pocket of the pink dress, which represents "happiness", to receive a key. The brilliance comes if you examined all three when the room was first unlocked - Miki liked the blue dress (silence) and the green one (peace), but stated that the pink one was too gaudy and that it wasn't her style. Miki has depression.
Why does Saeki crawl on the ground in her monster form? Because she broke her legs when she escaped the hospital by jumping off the roof.
Miki had medication she takes for her depression, but mentions they don't seem to be working; in the remake, she can even toss them down the sink. In one of the endings, it's revealed that Miki is very likely suffering from schizophrenia. Some antidepressants can actually worsen psychotic symptoms in people who are simultaneously suffering from schizophrenia.