Mother Jones has an illuminating interview with a friend of Loughner's:

Tierney believes that Loughner was very interested in pushing people's buttonsand that may have been why he listed Hitler's Mein Kampf as one of his favorite books on his YouTube page. (Loughner's mom is Jewish, according to Tierney.) Loughner sometimes approached strangers and would say "weird" things, Tierney recalls. "He would do it because he thought people were below him and he knew they wouldn't know what he was talking about."

And:

Since hearing of the rampage, Tierney has been trying to figure out why Loughner did what he allegedly did. "More chaos, maybe," he says.

"I think the reason he did it was mainly to just promote chaos. He wanted the media to freak out about this whole thing. He wanted exactly what's happening. He wants all of that." Tierney thinks that Loughner's mindset was like the Joker in the most recent Batman movie: "He fucks things up to fuck shit up, there's no rhyme or reason, he wants to watch the world burn. He probably wanted to take everyone out of their monotonous lives: 'Another Saturday, going to go get groceries'to take people out of these norms that he thought society had trapped us in."

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