Like you, I've been pondering random thoughts and have had trouble bringing them into some sort of cohesive form. How better to terrorize than to undermine the public's confidence in their local LEO's? By moving back into the D.C. area, now he's taking on that military "eye in the sky".
From all reports, his note threatened school children. It was pointed out this morning that this morning's shooting happened within sight of a school bus stop. I think this was an "in your face" effort to prove how easy it would be to fulfill that threat.