Well it depends from person to person I suppose. Video games which depict violence could easily penetrate juvenile minds and cause a change in their behaviour. For example, I have a friend who owns a gaming cafe and it's frequented by people of all ages. There was a time when I used to see teenagers coming in and mouthing off profanities for every wild stunt they pulled off in GTA: San Andreas. Some would go to the extent of hitting the monitor screen on loosing a game. I'm not saying that these people who turn out to be violent savages later on, but it does have some effect on the youth crowd who get so wound up in a game, that they don't know when to pull out of it.
Back when I was a teenager, me and my brother (and a whole lot of other people) used to be ardent followers, nay worshippers, of WWF (now know as WWE). We used to love it so much that we would literally sculpt those Rock's Brahma bull tatoos on our arms, get those fancy merchandise, cards and what not. At one point we all got so involved in it, that we actually began to pull off those stunts on our own. I remember this one incident when I tried to deliver 'Tombstone' (Undertaker's finishing move) to my brother and ended up momentarily screwing up his neck. It was then I realised why they give a huge warning: "DON'T TRY THIS AT HOME!" It's because someone is bound to get hurt. My point is that this aggressive show, which was supposed to be sports entertainment, ended up influencing us enough to try out something which we never would have thought of doing in the first place.
Violence is fuelled by aggression. It's in our nature. Some of us might have great self control to pin down our aggressive side, but there is a time when it you will let it loose and trust me, it won't be a pretty sight. Video games is just a one of the modes of depicting violence. Does it create a violent person? May be, may be not. Violence doesn't necessarily involve gunning down someone after playing GTA games. It could be anything. Swearing, entering into fist fights, random brawls, vandalising property etc. Each of these acts "could" potentially be the influence of violent video games. People who are naive and can't differentiate between virtual and real world are easy targets.
Video games have the power to influence people these days, just like movies and other mass media does. We may view it as a form of entertainment, play the game and move on, but for some it might have a strong impact. So it's best to balance one's life with some other activities.