Garfield's and McKinley's assassins may have been crazy (and they were working alone as far as I know), but they also had specific political motivations which made them part of larger movements.
In particular, Garfield, just like Lincoln before him, was assassinated precisely so that his Vice President would become President. In the case of Lincoln, his VP Johnson was a Union Democrat who might have been more sympathetic to the South than Lincoln would have been; in the case of Garfield, his VP Arthur was a member of the Stalwarts, the opposite faction to Garfield's within the Republican Party.
If their assassins weren't crazy, then they were stupid, since both of these reasons proved foolish. Lincoln was much more sympathetic to the South than the Radical Republican Congress, which kept Johnson out of the picture by impeaching (though not quite convicting) him; assassinating Lincoln did more harm than good for the assassins' cause. And Arthur made a point of refusing to implement Stalwart policies (mostly having to do with civil service reform, which is why nobody writes time-travel sci-fi thrillers about it), so assassinating Garfield had basically no effect.
On the other hand, assassinating McKinley was not intended to have any direct effect; the idea there was just to make powerful people afraid of random assassination attempts. You don't need a conspiracy when that's your motivation! That sort of thing ever does any good either, but it was all the rage in Europe at the time. In fact, it was still going on as late as 1914, as any Diplomacy player should know!
In answer to your question, Tolstoy, here is a list of assassinations and assassination attempts in the style of McKinley's assassination:
http://en.wikipedia.org/wiki/Propaganda_of_the_deed#List_of_assassinated_important_figures_and_other_propaganda_by_the_deed_acts
Most of these were, by the official story, done by lone nuts. Nearly all of them took place in Europe.
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