Legal, unless its material is of genuine depictions of otherwise illegal actions, eg child pornography, rape, torture, murder (snuff films), etc. There isn't anything wrong with instituting warning mandates and restrictions on its audience though, such as by age.
Urban legend, don't exist. No law enforcement agency in the world has ever reported finding one.
Actually there was at least one case involving a Russian ring awhile back.
Moral issues aside, a snuff film seems like it would be inconvenient to make. You would have to get it done in one take or not at all.