Do video games cause people to be more violent than they already are? Probably not.
However, media (games, TV, films, some music) does tend to glorify violence, normally under the guise of 'he's a good guy' and make it look pretty damn easy. Now, that's not to say it'll make your typical person want to give it a try - but those who are unstable may well be affected. Again, this is true of all forms of media.
Is violent media a problem? Most of the time, no. However, I believe there should be some form of responsibility to keep it in check: making sure violent programs are past the watershed, enforcing cinema ratings, selling video games to people (a) of age and (b) disclosing why it is rated.