Wikipedia says it pretty well : "was much criticised by those who believed it exploited the subject of presidential assassination, and that by doing so, was in bad taste"
There's a quote by Hillary Clinton, like it or not, likely to be our next president, describing that movie as "despicable". I never thought I'd agree with her on absolutely anything, but here I am.
(and again I state two wrongs don't make a right, etc)
FWIW, I did see it at the time, and thought it was a fairly intelligent and sensitive depiction of what might happen to American society if Bush were murdered. In any case, why should such a "what if" scenario be off-limits to artists?
And she's a despicable person. Sadly, she's probably going to be the best candidate in the next election, but she's a horrible person whose opinion counts for nothing.
http://en.wikipedia.org/wiki/Death_of_a_President_%282006_fi...
I'm sure there are earlier precedents too, but this one comes to mind immediately.
Oh, another one: Team America: World Police... which actually did depict a mission to go kill Kim Jong Il (while he was still ruling North Korea). Though to be fair they didn't actually manage to kill him, since in that movie he turned into a cockroach and fled in a spaceship.