Consider the observation that many school shootings have been perpetrated by people with a history of playing violent video games. In this scenario, what could the third variable be?

The right option is "the shooters history of being bullied"

Explanation:

People with a history of being bullied turns out to be bullies or portray a high level of violent behaviors. There have been studies that shows a correlation between playing violent video games and violence. There have also been studies that suggest that people who have a history of being bullied will grow up being bullies and displaying several violent conducts.