Anything can 'cause' violence. Some guy might get up late, spill coffee on his pants, get mad, drive to work mad, get in a wreck, act an ass, and get in a fight, somebody pulls a weapon and somebody dies... and it all started with just a bad morning, and might not have happened otherwise.
Violent video games can be entertainment for some, a safe outlet for others, or an exacerbating factor for those who are already "borderline personalities" I don't doubt. The same could be said of violent movies and violent music.
But I don't think they are usually the PRIMARY causal agent... When I was a kid we watched war movies, cowboys and Indians, martial arts movies... then we went outside and played out those kinds of things with our friends in make-believe... but I don't think they actually made us significantly more aggressive or violence prone in and of themselves, and I'd say the same about violent video games and MOST young people.
But there are always exceptions. Someone who ALREADY has "issues" might find violent video games to be either a safe OUTLET, or an exacerbating factor, depending on circumstances.
Sometimes when I'm fed up with the human race an hour or two of "Destroy all Humans!" is just the thing to take the edge off my anger and frustration in a harmless way.
OTOH someone who already has a broken conscience and damaged "governor" might instead be inspired... but then again such a person might have gained inspiration to violence from any number of sources.