You left out Garfield. His assassin Guiteau was ostensibly a Republican but one with mental problems. His family tried to get him committed in 1875. He convinced himself he got Garfield elected and deserved appointed office. When that didn’t happen he snapped.
I was going for politically motivated ones, even if crazy.
But, yeah, I forgot Garfield.
I also debated adding Gov. Shapiro, as the killer was motivated by Shapiro not towing the Democrat line on death-to-Israel, but he wasn’t federal.