If the reporting I saw can be trusted he had a counselor assigned to him personally the entire time he was in high school. He needed a lot of attention and they knew it and dealt with it. However when he left high school that support system disappeared. His mother (who reportedly was afraid to leave him alone) tried to keep him stable and attempted to integrate him with society but clearly it didn't work out.
Just in that report that I saw there were so many warning signs that this kid was a loose cannon and many people knew it. His mother was probably in denial hoping that she could make him turn a corner. She obviously needed help from a professional for him.
But back on topic, games aren't the problem for most people but for this kid who may not have been able to clearly define the boundaries between fantasy and real life they may have been a contributing factor. That said, we don't all need to be treated like we are mentally ill.
Maybe the video game rating system needs to put a reminder for parents that a violent game is not suitable for mentally ill or potentially unstable youths.