The assassination of James Garfield and attempted assassination of Ronald Reagan are both very clear evidences of "weird dudes acting alone." I don't know enough about McKinley's assassin to know if he would also qualify, although that was part of an era where anarchists assassinating notable political figures was relatively common. I also don't know enough about Teddy Roosevelt's attempted assassination.

Indeed, if you look at attempted or successful assassinations of US presidents, excluding JFK, only Lincoln's assassination involved a conspiracy.