usually [libertarians] argue that possessing footage containing murder, adult rape or any other crime really is not currently forbidden
which I assume means
libertarianism concludes that possessing footage containing murder, adult rape or any other crime is not currently forbidden.
I'm not aware of libertarianism concluding this. Could you explain how it concludes this? How does it go from the non-aggression principle to the nonforbiddance of possesion of records of violence? It may be that libertarianism does not forbid possessing records of violence, but there may be some other systems in the world which do currently forbid it. For example, it is forbidden to possess real (as opposed to drawn) depictions, a sort of record, of a real child being raped. This is a special case of possesing real depictions of a child participating in a sexual activity, which is forbidden in many jurisdictions.
The only thing forbidden by libertarianism is aggression. If it's aggression, it's forbidden, if it's not, it is allowed. (Violence for the sake of restitution is allowed, but if the violence is excessive, it's considered aggression.)
Other things may be forbidden, for example, by a powerful aggressor such as a legal system (an instance of illegitimate forbiddance), or by one's promises or agreements (an instance of legitimate forbiddance).
Perhaps you meant that libertarianism does not forbid the posession of depictions of aggression. It doesn't. If anything, it encourages it, for the sake of freedom of information, which is important in upholding a free society. Banning the possession of some record of aggression would make it more difficult to help the victims extract restitution from the aggressor.
There is nothing inherently harmful about posessing a real depiction of violence, murder, rape, child sex, aggression, or whatever. There may be cases in which a person, having such a depiction, gets the desire to commit such an act. If the act is not aggressive, then there's nothing wrong with his doing it. If the act is aggressive, it's not the fault of the depiction, it's the fault of his psychical disorder which makes him want to do something aggressive because of a depiction of aggression. He should seek psychological help, or risk being aggressive. If he does do something aggressive, he'll be the one to blame. Probably, his insurance company will be able to recognize individuals who are at-risk for such psychical conditions, and may offer them a discount on their insurance premium if they get psychological therapy.
My guess is that you're not going to read all this. The fact that you have such misconceptions indicates that you did not learn about libertarianism before responding. That's too bad. I hate to see more another idiot increasing his idiocy.