Continuity mistake: When Grace and Ms. Mills are sitting in the living room you can see a clock on the right, but in the next shot it's disappeared.
Continuity mistake: Before both kids go out by the window, Anne is painting, and the paint shows different patterns in each shot. (01:21:25)
Continuity mistake: When the father comes home, the scars on his face are very small - in the bed scene they are a lot bigger. (00:55:45 - 01:04:20)
Continuity mistake: Anna's eyes keep changing throughout the whole movie. One scene they're brown, another they're blue, then they go to brown again.
Continuity mistake: When Grace is explaining the children's photosensitivity to Mrs. Mills, she lights an oil lamp, yet a large shadow of the picture frame then appears directly above the flame. This would seem to be cast by a handheld lamp somewhere off to camera right.
Continuity mistake: When Mrs. Bertha Mills gives Grace her pills, she first sets them on the table, and we can see that the pills are about 1/2 inch apart. Then it cuts to a closer view, and the pills can be seen to be touching each other. Then it cuts back, and the pills are apart again.
Continuity mistake: When Anne and Nicolas are at the dining room table having soup, you can see that Anne's spoon is next to the bowl and not in the soup itself while she is reading and not eating. A few seconds later she still has the book in her hands, but now the spoon is in the soup. (00:24:05)
Continuity mistake: When Grace opens the kitchen and all three head in, Mr Tuttles is on his side. When it cuts to him coming through the door his right arm is up and on the door frame.
Answer: Evil and good do not come into it at all. They are simply dead people. As Mrs. Mills says it, sometimes the world of the living gets mixed up with the world of the dead. They simply died in that house and can't leave just like Grace and her children.
Garlonuss