It's interesting that this thread has, essentially become,
1 - "I'm scared of being robbed, invaded, set upon by my own government, so I need to buy a gun. Now I feel safe!"
2 - "I'm scared that you own a gun, because, let's face it, I don't trust you, or your child rearing techniques. Now my options are to belittle you in to giving away your gun, or get a gun myself."
As for the original topic, was he gunning for the congressman? Probably. Chances are he wanted someone to see him, and what better way. Though "probably" isn't fact, and hind-site changes original intentions, so no one will probably ever know.
Could an insane person be spurred on by another, sure. Most of everyone is looking for a guiding voice of some kind or another. And talk radio "entertainment," and the current political "discourse" are as good a place to get a lacking parental figure as any. Does that mean shock jocks (is that even a term anymore?) and politicians who do not pay enough attention to the impact of there words are to blame, no. These people have the right to say what they want; and so long as we are living in a culture that actually thrives on, and is driven by fear, these people will have an audience... not to mention some coin to line their pockets.
In summary, Caucasian middle-class Americans seems to be the most well to do terrified bunch on the planet. And by gum, I love 'em. They sure do make a web forum an interesting highly opinionated read at four in the morning!