Most people believe that violence and aggression is antisocial and harmful.
We have psychological and socialinhibitors against using aggression to resolve interpersonal conflicts.
However, inhibitions are loosened after exposure to violent media.
Therefore, agggressive behaviour becomes normalised, and the more we expose ourself to media, the more our norms go from non-acceptance to acceptance of aggression in certain situations.
Disinhibition is more likely if, in the media, the aggression is justified, the effects of violence on victims is minimised, or if it's rewarded and made to appear normal to everyone.