Posey killed because he had an extremely abusive family.
I know because Slaughter Ranch is on the same road as my familys and my family knew him.
I watched a lengthy show about this trial (maybe was Dateline or 48 Hours) and it honestly had me feeling horrible for this kid. The show featured numerous people who said they knew of the horrible abuse he suffered, and did nothing about it and now regret that. The father's computer showed him researching incest, among other horrible things. And there was a situation with his stepmother that was just gross.
I don't much about the video game deal, but the abusive situation sounded horrific.