There have been evidence of snuff films, there was a huge Russian based child-porn ring/website broken up a few years ago called "Wonderland." Where for the right price they would kill the kid and film it, and the perv would watch it. The police have evidence of these digital tapes, and part of one was shown on an Italian news broadcast!
BTW you
contridicted yourself when you said "...Some serial
killes have
recored themsleves torturing and killing, but these crimes were not filmed(???)..." And what the fuck is a legitimate murder?