For one he is a left wing extremists. He is a big fan of Obama, Biden, and Anderson Cooper. In fact he sent a package with a letter dated december 8, 2012 to Anderson Cooper. Anderson Cooper claims the package only arrived a few days ago. That would be the slowest postal delivery since the 1800s. Someone had to have known about this package and the letter in it, long before the present killing spree.
Furthermore in his manifesto he argues for an assault weapons ban. He wants happy guns banned and standard capacity magazines done away with across the US.
Perhaps that is so his war will be easier to wage.
I bet that there are a few more recent shootings involving left wing extremists as well. These people are such true believers in their cause they are willing to ruin their own lives to ruin the lives of millions of american citizens who own guns.