Manson isn't a leftist. He hated his fellow hippies (except the Beatles, of course) and the crimes he and his followers committed were about insanity, not politics.
The only reason Squeeky tried to kill President Ford was to keep Charlie in the headlines, because Charlie is a narcissist, among other things. I don't think either one thought that killing President Ford would actually result in Charlie's release from prison at that point.
If Manson and Squeeky are leftists, then the right has to claim John Eric Rudolph and all the allegedly pro-life activists who have burned down clinics and killed doctors.
The line between leftwing politics and insanity has always been blurry. he Unabomber and Al Gore held remarkably similar views, and Marx's apocalyptic vision of the collapse of capitalism was based on his own sense of drama and rage at being expected to repay his debts, rather than any scientific analysis, and it appeals to like-minded narcissists who see themselves as the saviors of mankind.