So he was raised with the idea that he can get whatever he wants, whenever he wants. If he can play with his favorite things, guns, at age 8 and play tennis against champions at age 11, what kind of crazy bitch would dare to reject his advances? He even waited for the New Year's party so it would be as romantic as possible. If that's how this kid thinks, it's his father's fault for spoiling him, and his father's fault for giving him access to guns and supporting his obsession with guns. If the kid was unstable, the dad should have paid more attention instead of just showering him with money, gifts, and guns.
The kid had obviously been planning this for a long time. He wanted revenge, because this was the first time he didn't get what he wanted. He wanted to prepare for months. So he started doing research online about school shootings, for weeks prior to the shootings. He had an exact amount of targets and made sure to get headshots to stay fast and efficient, so he wanted to learn as much as he could about other school shootings. He was a coldblooded killer and he wanted to be. Games had nothing to do with this. Without games he would have just practiced with his real guns anyway. The fact that he played a game on his last night for only 2 hours is not a more important detail than the weeks he spent researching school shootings, or the 9 years he spent shooting with real guns. It is an interesting detail, but it should not overshadow the rest of the story and make this a "blame the games" story.