why do dead people suddenly becomes good?

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I read sometime ago about this serial killer who was caught, and asked why he killed so many people and would even in to their funeral. Much to the polices surprise he admitted the crimes and told them he liked killing there people who if thinks are bad people because in the eulogy, or last words by their family and friends they become good people, and on matter how cruel they were not a single of those mean things were said. This makes of think, well yes, i wonder why this is the case. Why Does people always say only the good things about the dead? Do we do this out of respect for the dead, and well are there anymore reasons why?