You can hold a genre such as the Roth/Tarantino "sicko-pact" spree responsible for grinding violence into the minds of impressionable young people. They may not go and start killing people, but most certainly it will play a part in moulding some of them into becoming more aggressive and less compassionate human beings as a result.
TV influences people. IF it influences people to buy things they don't need, to beleive lies, to live their lives according to materialist values, to conduct their social interactions in many different ways, then why not also to kill people?
People start acting like characters they aspire to be from their favourite shows, I've witnessed myself do it unconciously. IF it is capable of homogenising and mediating society to an unhealthy playing field, is it also not capable of isolating people and turning them into sociopaths?
You have to look back further than the start of the film.
Individual circumstances are by far the most influencial factors in the creation of the psychopath of course, but the evidence is still there there that sometimes people do go nuts and copy violence from films so you cannot say films are beyond reproach.
Sometimes people just do it out of sheer badness. When A Clockwork Orange was re-released in 1999/2000(?) following the death of Stanley Kubrick, in the town where I lived at the time, in the outskirts of London, where as it happens Kubrick also filmed 2001, a man my age was attacked and beaten by a gang of 14-18 year olds outside the cinema, on the night or around the weekend it was re-released. This is a regular occurence in that town, in fact when I lived there I was scared to go out the front door.
You can't hold a film wholly repsonsible for a copy cat killing though, but you can hold it responsbile for influencing the method of killing. That does not incriminate the creator of the film though, he is not directly attributed to the crime, he just thought it up, the copycat was free to choose a different method.
It's art reflecting society, but I am not ready to completely discount the possiblity of there being some sort of a cycle if destruction going on between the two, art depicting society and every other time just a little bit more influencing the behaviour of society back.
"...people were sadistic before they invented the sadistic film."
Not always. Whilst documented history contains countless examples of war and fighting, never at any time has violence been so globally pandemic to the extreme levels we are witnessing today. What concerns me is what effect this kind of media violence will have in years/decades to come. Besides societies current trend towards ever more destructive values, and violent behaviour, there is also this new medium influencing society in many other accountable ways.
A couple of examples.
A Clockwork Orange (film - Wikipedia, the free encyclopedia)
(section British Withdrawal)
Sorry your only getting one link, I can't find a source for the Child's Play 3 copycat killing where two young boys stole a baby and placed it under the wheels of a train in around 1993 I think that was.