In Raman Raghav 2.0, The movie starts with these lines which tell a killer's story.
And then this line appears it and says "This film not about him".
Then why do they show unrelated lines at the beginning of the movie?
Because it was not so unrelated. The film's killer was based on the real life killer and even Anurag Kashyap originally wanted to make the film on real killer only:
Kashyap told that him that he'd wanted to make a film on the notorious Raman Raghav, infamously known as Psycho Raman, but the budget constraints did not allow him to make a period film set in the '60s. Even the title of the film Raman Raghav 2.0 came into the picture even before the script was ready. - scoopwhoop.com
So due to money issue they made it set in current setting and made the killer fictional. Even film's killer Ramanna acknowledged the real life killer.