Mr. Abdo apparently was unsatisfied with that level of carnage and wanted to take a second crack at things, including possibly using a bomb as well as guns. Furthermore, as with Hasan, there can be no doubt that his religious beliefs played a large part in fueling his rage. Of course they also allege that Abdo had child pornography on his computer, which exposes him to charges of hypocrisy, but then again, huge numbers of religious extremists are hypocritical and have double standards. It doesn't appear to make them any less extremist.
Like Hasan he believed that a Muslim soldier should not have to serve in the military when it is engaged in operations against other combatants who are also Muslim. Well gee, christian soldiers have had to fight against other christians in almost every military conflict the US has ever had. Why should there be a special exemption when you're a muslim?
Naturally, logic and careful thinking are often severely lacking in the minds of religious extremists.