The problem I have with the discussion and the reason why I don't now and never will buy the link between violent media and atrocities such as this, even when it's placed under the banner of "possible", is because violence and violent people have always existed, video games, movies, music, the written word, hell art of any nature have not.
There are psychological and physiological reasons for increased aggression leading to violent behaviour and by saying that this is "possible" is legitimising the argument of people who are for banning it and taking attention away from discovering the real cause.
I know that I'm going to come across as maybe being ignorant and dismissing other people's views because they contradict my own but I can assure you that's not the case, I believe what I believe because it is the most logical and reasonable view and from my own personal experience the one that is actually supported by real evidence. I know a lot of people, of those people I would say they have all seen a violent act portrayed in some form or another and none of these people have ever committed murder and the vast majority have never even been involved in a confrontation where they were the aggressor. I personally have been watching violent movies, playing violent video games and reading novels including violence since a young age and I abhor real violence and the people who enact it.
The point that I'm trying to make is that violence has always been around, people are not inspired to commit it because it's a part of who they are, whether that be because of childhood trauma, psychological deficiencies, chemical imbalances etc and instead of focusing our attention on discussing a flawed argument we should be examining violent offenders and comparing them to non-violent people so that we can finally take a step towards preventing these tragedies rather than running round in circles with our heads up our arses.
Also, as you mentioned in the video the fact that this scumbag dressed up and claimed he was the Joker does not show that he was inspired by the film, people who commit these acts want attention, it's the same with serial killers wanting to be caught so the world acknowledges their work or release manifestos, it's attention seeking. A random shooting in cinema makes the news, a shooting at a Batman screening by a person claiming to be the Joker gets the world talking.