I don't dispute your second paragraph, however I believe that there have been examples of what would appear to be politically motivated killings
Richard Poplawski kills three police officers in Pittsburgh, convinced that Jews controlled the media and
President Obama was scheming to take away his arsenal 4/09
Jim Adkisson opened a guitar case at a Tennessee church pulling out a gun, killing two, and wounding four more.
He targeted the congregation out of hatred, saying, “liberals should be killed because they were ruining the country.” 7/08
Timothy Johnson shot and killed the democratic party chairman of Arkansas, Bill Gwatney, 8/08