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.
I think your confused. The abolition of slavery was a right wing thing.
The want to have a slaves was left wing
No. “Republican” does not equal “Right wing”. Maybe in the modern context, but political parties evolve and shift on the left right scale over time. The Republican party was originally established as a less Conservative opposition to the Democrats.
Besides, the policial party pushing for the thing is surely much less important than the thing itself. The act of enslaving a portion of the population, based entirely on racial divisions, is an extremely right wing position.
Ironically it’s also extremely left wing position.