It won't cause a violent person to commit an act, but it most certainly desensitizes them to it. In fact, pretty much anything we see, feel, or experience over and over again desensitizes us to it. And if you want the scientific explanation for that, consider that our emotions and feelings are nothing more than chemicals being released by our body into our blood stream and brain; and pretty much any chemical that produces an acute reaction will desensitize someone to it over time.
Look no further than popular media:
Why do you think that people confuse love with lust? Sex is all around us, and all we've ever heard love and sex being used as words to describe pre-martial lust and animalistic behavior, instead of what love and sex have historically been about: pro-creation and a complete and utter commitment of dedication to the other person (agape love). Now it's about how long and how often and how young we can **** a person before we break up or divorce them.
Now, even worse, they're associating love with pain, rape, occult rituals, bondage, and murder. It used to be about domination of men and submission of women (see Playboy, or the sex kitten/sex bunny/pre-nubile craze of the pre-2000's, and so on); that's not what love is actually about (and how many women will just let you walk all over them?). But now... Ever watch Lady Gaga's "Telephone" video? What about Christina Aguilera's "Not Myself Tonight"? What about Rihanna's "Disturbia"? All it really takes is cognitive dissonance: associate a woman with a sex kitten object, then in the next generation associate a sex kitten object with having a ritualistic orgy in a Church (I'm looking at you, 'X-tina'). Pretty soon Love and Life will be synonymous with Pain and Death.
People who see something distorted from a young age have a distorted sense of reality; this is all over psychology. It's a slow but subtle change that can't be noticed in our life time, or heck in the lifetime of our parents. And it's impossible for the person themselves to notice it, since their reality is distorted. And see Cognitive Dissonance for how it can even make a big difference in your life time too.
Listen to music in the 30s, then 40s, 50s, 60s, 70s, 80s, 90s and then modern music, and tell me you don't notice a difference in the subject matter. Then the society around us and the parents who bring us up and our peers and friends and the school teachers and coaches and so on and so forth all reinforce the messages we hear with their actions or words.
Now, apply this to violence; in the media, in our games, in our lives... it's a vicious cycle. No, it doesn't create violent 'people'. It creates a violent society that takes guns and knives and fifty friends to fight their problems instead of working it out in a civil way. And no, I'm not talking about the civil way to work out a fight ten years ago, or twenty; I'm talking about the civil way to solve problems a hundred years ago, when neighbours could walk up to each other and talk in a civil manner, or politicians could speak one to one without insults and berating each other, or coaches and parents could resolve issues with the referees without yelling and screaming, and so on.
Funny, because they were right. It just skips a generation or two until it's ingrained enough in society that nobody notices it's any different. Once we fail to teach our children proper values, or the values we try to teach them gets drowned out in the sea of other messages they hear, our kids can't teach their own kids any different and society is forever damaged.
The moving picture is now the only thing that society pays any attention to, and it's damaged our ability to communicate with others and pushed messages of sex and violence into our heads since we were old enough to see the messages, as did comic books, but more so sex and male domination for comics. Drugs have now infected society, particularly the rock and roll and hippy side of it, hence the 'War on Drugs'. People are starting to get into cults, skulls, death, and more: see the popularity of the Twilight novel.
You need to look past what we see now and the acute effects on people now and examine how society has changed over generations. I can tell you that my friend has an eight year old brother who thinks shooting up cops is the funnest thing ever because he plays GTA all day, and that all he ever talks about is 'blowing the **** out of stuff'. Sure, that's a parent thing, but they're also immigrants, so what do you expect? He also has no friends and no life, so what do you think will happen in ten years? He'll grow up into a functional human being? No.
I could go on and on and on. But no, 'bad' things don't necessarily create 'bad' people. It's when the entire society has been infected by it that we create 'bad' people; that could take a year, or a decade, or a century, but once people get into it at a young age, it's all over.
EDIT: That's my rant for the month.