JMO, I think if we knew everything there is to know about AL and why he did what he did we might find that there is not any one thing to blame, there are a lot of things in combination to blame, some perhaps more obvious than others. Obviously guns gave him the means to inflict maximum suffering on a large number of people and if he hadn't had access to guns he might not have been able to kill as many people but access to guns probably wasn't what made him want to kill in the first place. Maybe videogames weren't either but it may have been a factor in what kind of attitude he had to violence. There are some scientific findings that indicate that exposure to violent games and television/ film violence may e.g. increase hostile expectations and desensitize us so that our brain responses to violent stimuli change.
And yet I can point to just as many, if not more studies that cannot find conclusive evidence that violent games (or any games for that matter) is linked to violent tendencies in RL. There is a reason why the Supreme Court continually strikes down these lawsuits wanting to ban violent games, because no one can come up with proof that they are harmful, just like you can prove that an R rated movie is harmful. everthing should be done in moderation, but that's not limited to video games.
I've been playing games, violent or otherwise, for the better part of 20 years. I'm in my 30's now. I think I turned out ok. I'm not a violent person, I'm not even a gun owner. There are millions of people exactly like me, in fact, older than me. I think many people would be surprised how many people who are not a teenager play video games. Like for example, is it known that the average age of a gamer is 28?
If your a parent, it should be on you what your child does and doesn't do. I know some parents who have no issue having their kid play Call of Duty or Battlefield or whatever. I know others that don't let their kid play those type of games. I am fine with both sets of parents. Because it's their child and they know what's best for them.