Interesting choices. I think the only valid one there would be the second attempt on Gerald Ford (Sara Jane Moore), who wanted to spark a revolution.
The first attempt on him was by one of the Mason Family (Lynette Fromme), who were sometimes described as hippies, but were also keen on bringing about a race war.
John Wilkes Booth seems surprising to call left leaning, as he was pro-confederacy and angry about the abolition of slavery.
Lee Harvey Oswald was a Soviet defector, and did indeed call himself a socialist. However there’s the whole probable scenario that he wasn’t actually the shooter and never went on trial.
To mention other presidential assassinations, there’s Charles J. Guiteau, assassin of President James A. Garfield, who was a loyal republican, but felt he was personally wronged by the President (as well as mental illness).
Leon Czolgosz, assassin of President William McKinley would probably support your hypothesis more than the others, as an anarchist.
All in all, not really overwhelmingly left leaning.