The wife didn't find out about her husband's infidelity until the day she died unexpectedly. Even though she was full of anger and grievances, she had nowhere to vent. Even if she wanted to divorce him, she couldn't, so she had an "unfinished" feeling. Live on. When the dentist was hit by a car, that was when she truly forgave her husband. Perhaps facing the shocking scene, separated by life and death, she finally figured out, what can't be forgiven when everyone is dead.
PS: The whole movie is neither scary nor sad, on the contrary it is a very nice comedy, I really like the dentist's English accent, expression, and his attention to detail.
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