The most obvious negative result of violence in the media is that it promotes using violence to solve problems. By watching detailed violence, it sends a message that violence is acceptable as a means to an end. To make things worse, sometimes the violence doesn't have consequences. For instance, a cartoon character shot with a gun will be walking in the very next scene.
A viewer often becomes desensitized to violence by watching acts of violence continually. It takes a more horrible act in order to for the viewer to become aware that violence is occurring. This makes people more accepting and tolerant of violence in their real lives. People desensitized to violence will have no problem hitting their children because merely hitting is not seen as a violent act. It takes such acts as a shooting to shock a desensitized person.
People who watch a lot of violence in the media may believe the world is a dangerous place. This is known as Mean World Syndrome. They see the world as a violent and threatening place that one must constantly be on guard. This view is not only sad, but can lead to violent acts when this type of person reacts to a threat. He may perceive a slight where there isn't one. When you're always on guard, the world can be a scary place.
One potentially good side effect of violence in the media is the catharsis that it sometimes provides from watching it in such detail. Someone with violent tendencies can watch the violence played out on screen without hurting anyone and have his fantasies met. Cathartic viewing of violence is a great way for aggression to be spent, but one must take care that it does not spill over into the real world where that aggression can lead to hurting people. Although this is a positive aspect of media violence, it's still very dangerous in its own right.