Politically motivated shootings

What's up with these recent shootings?
First, this. A man enters a liberal-oriented church and starts shooting. He kills two people and injures others. He is arrested and tells investigators that he believes that liberals should die.
This second example is more recent, and the shooter was killed by police. I don't have a ton of information about it, and I'm not even sure what prompted the shooting, although I think there's a good chance that it's similar to what motivated the psycho in the first example. This time, the shooter entered the Arkansas Democratic Headquarters and shot and killed the state party chairman.
What prompts people to do things like this? Make no mistake, these terrorists were raised in the US. The first one killed for political reasons; the second one may have.
Is it cultural-based? Are they just plain crazy? Or is it a mix of the two?