Arthur Bremer, who stalked Nixon and shot George C. Wallace, was a Wisconsin liberal -- a fact I didn't find out about for more than 20 years after the event: the MSM covered it up completely.
LHO was a self-professed Communist, a defector who took US Marine Corps secrets to Russia and helped the Soviets shoot down Francis Gary Powers. He also tried to assassinate a conservative U.S. general with the same Carcano rifle he used to kill President Kennedy.
Lynnette "Squeaky" Fromme was a member of the Manson Gang and complicit in the Tate/"Helter Skelter" murders. She also tried to shoot President Ford.
The Unabomber was a PhD whack case at Berkeley and left-wing environut.
Loughner, Harris and Klebold (Columbine), and the Aurora shooter we know about.
The Virginia Tech shooter is a bit of a mystery (to me), but the Washington, DC, sniper was a leftist wacko who hated white people, as was Colin Ferguson, the Long Island commuter-train shooter who murdered U.S. Rep. Carolyn McCarthy's husband and kick-started her political career.
Patrick Purdy, the original 1989 schoolyard shooter (who used an AKS, prompting California's semiauto "assault weapon" ban), was a left-wing mental case and "raggedy man" with peace symbols on his clothing.
The Fort Hood shooter, Maj. Hasan, is a fuzzy-wuzzy jihadi who was recruited by al-Q'aeda.
Not all of the world's mass-murderers are left-wing, but that sampler seems to indicate that lefties are "overrepresented" in the ranks of the mass murderers of the world.
See I knew it.