Almost every species has it. Only sociopaths lack this violence immune system. Even with trained soldiers, only a percentage of them can easily bring themselves to actually kill in situations other than self-defense. The same factors are used in violent media programming. From young ages, kids are trained to accept violence as a natural part of life.
Cartoon characters batter each other.
In the early 1990s The Journal of the American Medical Association published a definitive study on TV violence. Comparing regions with television to regions without, and keeping most other factors the same, it was found that in every society where television was introduced, there was an explosion of violence on the playground. In control communities observed during the study, there were no such changes. Children come to associate violence with entertainment. They eat and drink while watching, so that violence becomes part of a pleasing routine.