"Is it illegal to kill a person if she has no family or friends?"
Anyway, the question here isn't whether you get caught. The question is whether it's actually illegal if there is no copyright holder that anyone knows of, and one doesn't come forward with a legitimate claim. Will law enforcement charge you with a crime?
I'm not a lawyer and have no experience with law, but I do love free (and legally so) movies, so I'm genuinely curious about this.