There is a difference between an actual murder and a depiction of a murder. In the former, someone actually dies. In the latter, an actor falls over.

The actual crime is very much an element to the illegality of a snuff film. Snuff films are illegal precisely because someone actually dies. Child porn is illegal because a kid actually gets raped.

There are a few rare situations involving "fake" child porn. Should these be illegal? The Supreme Court has said that virtual child pornography might not be illegal but that either way such content was justification for holding a person in a mental institution as a danger to others. There is an extremely high (as in, greater than 80%) correlation between willing viewership of child pornography and attempts to abuse/molest/rape a child or acquire actual chil porn. With pedophiles, child porn isn't just an abstract media form like a TV show; it's like going to a Ku Klux Klan rally with a snuff film showing a black man get lynched and exhorting the crowd to do the same.