Re: No, it doesn't point towards a serial killer.
There was a Vietnam movie where death cards were placed on the dead VC.
It was the unit’s “calling card”.
If there was any connection, he's seriously into role-playing and could have twisted the role of a First Person Shooter PCgame, with highly realistic effects, into his own personal campaign.
The level of disregard for, and total absence of any value place upon human life is reminiscent of school shootings.
It could be one or more kids playing a game.
(Just speculation)