Since 9/11, have right-wingers killed more Americans than the terrorists have?

The battles with terrorists in France this week and the countless condemnations of Islam prompted one liberal American pundit, Sally Kohn, to make this incendiary assertion on Twitter: "Since 9/11, right-wing extremists (including anti-abortion, anti-gov) have killed more Americans than Islamic extremists."
Sounds a bit absurd on its face but PolitiFact took a look and found that there's quite a bit of truth to the claim.

Statistics compiled by the New America Foundation, a Washington think tank that promotes data-driven policy innovations, show that in the time since the 9/11 attacks, jihadists have killed 26 Americans on U.S. soil, while those with right-wing leanings have killed 39. The single-most deadly event by an Islamic extremist was the 13 people killed at Fort Hood. On the right-wing side of the ledger, the worst was the six people slain at a Sikh Temple in Wisconsin.
But the categorization of these killings is complicated and a bit dicey, which prompted PolitiFact to deem Kohn's claim "Half True."

Here's a bit of the fact-checkers' analysis:
"If this exercise shows nothing else, it is that the number of post-9/11 deaths in the United States from either cause is low, and drawing firm conclusions is dicey. A single event or a change in definitions can shift the balance.
"The matter of definitions makes a big difference because most of the killers acted on their own. Experts in terrorist and extremist violence told (sister website) PunditFact that in these cases, it can be difficult to draw the line between ideological and purely personal motivations."

Among those PolitiFact interviewed was Alex Schmid, a research fellow at the International Centre for Counter-Terrorism in the Netherlands.
"Mental illness plays a role in up to 40 percent of the lone wolf attacks," Schmid said. In contrast, he said most organized terrorists are "clinically normal."
Clinically normal? Maybe by way of a medical definition. But I'm sure the people of France would tell you the terrorists are definitely crazier and scarier than those with mental health problems.