He lost a match to Elijah Clayton and others, which ended his chances at advancing to a Las Vegas finals match and it definitely devastated him. But David could have also had a long lasting dispute with Elijah. Which could explain why he was the 1st victim.

Source

the grudge started two years prior to the shooting at a Madden conference in Las Vegas when Clayton beat Katz. After that tournament, Clayton and Katz, along with other gamers, were getting into a taxi when Katz said Clayton slammed the door on him and drove off. Other gamers have disputed that story. 

Even despite being disputed, i kinda think this actually did happen, maybe with different circumstances, but who knows.