More important is the question, "Which identifiable group in the US is the most violent?" It's not a beauty contest between right and left, it's first a police and security matter and second it's a question of trying to identify if there are things to be done to make that group less violent.
Narrowing it down to right and left ignores the loons who are on either side because they have to be somewhere, as well as those on neither side. A lot of people "seek" some group to validate them, to say that they're right, whether because they're mentally ill, from wrecked homes, broken marriages, some sort of abuse. Their affiliations are personal and not political, whatever the groups they may be in. I was in a church that had 'seekers' come and go; they came not because they liked the doctrines, in fact they usually accepted them temporarily or marginally. No, they were after validation and support.
It's comforting to paint McVeigh as Xian because then he's in a group other than "ours." As though that were the chief point. "Well, then, what he did was horrible, but what's really important is that I can't be tarred with the brush that tarred him." We then get to decry the people we always wanted to decry. The various Muslim wanna-be homicidal maniacs aren't "right" or "left" by any usual metric and, because we want to avoid the taint of "Islamophobia" we find justification, a way to blame those actions on those we don't like.
Loughner was all over the place. It's been claimed he was well left of center before 2007, just as more recently he's been with groups right-of-center. People change, but he was weird then, too. And last fall, saying that a woman who got an abortion was a terrorist? Few groups that 98% of conservatives would recognize as "conservative" would go that far. And then there's this,
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x4689670, a link to a racist group. Broader labels are harder to make stick. A narrow label is easier to make stick, but just because we can assign him a label doesn't make it any easier--there's no evidence he joined the group, he just liked it. (I think back to the guy who joined a pacifist 7th-day church because one of its members, his acquaintance, was a big believer in zero-point energy, something unrelated to the church as such.) Similarly, a relative was diagnosed with calciphylaxis; a lot of people breathed a sigh of relief because it had a label. The doctors didn't; not even its symptoms are treatable. The label was, ultimately, meaningless.
I don't know if Loughner is right or left or if those labels are just a matter of our convenience because of some group he found some link, some hook, into. If we assume his target was Gilford, then we're happier than if we assume his target was Roll; if his target was Roll, we're happier thinking he was targed because of an immigration suit that went "our" way than if he was targeted just because he was a federal judge. We like saying Palin put a target over Gilford; we dislike saying Kos put a target on Gilford.
I assume that "right" and "left" are less important than figuring out why this guy bounced around, figuring out how to spot people with this kind of need, how to identify people who are going to snap and getting them the kind of help they need. I'm less concerned with making sure that his path to deviancy avoided any path I was on; with making sure that the vitriol and hate, if that was what actually fed his deviancy, was only of the sort that I like to deride as vitriol and hate. One saves lives; one salves consciences.