"I heard most of time this kind of crimes happen as revenge." I wouldn't say "mostly" these kind of crimes happen as revenge, a human would definitely not kill another human to this extent, deadliest, as a revenge, they'd usually just kill him. This really has to be a personality disorder. Its like the killers' unordinary fantasies! The victims above were killed by people who had narcissistic personality disorder, the definition really relates to the pictures, they have fantasy of killing this way, which is totally bizarre personal disorder. This is really far than the situations back in our country or mid-east or wherever. This is different!