Donald Trump loves a conspiracy, and he doesn't care how little evidence there is to support it. The Daily Show piece is a good survey of this:
President Obama had a few good lines about it as well. (There is some speculation that Trump's humiliation at being trumped by facts led eventually to his current run for the presidency.)
But as fun as this is, there are humans at the other end of his loutish idiocy. This week he claimed the Clintons had their friend Vince Foster murdered. Five investigations, Jake Tapper of NBC News, and the Daily Show prove this to be a lie, which will only deter Trump from repeating it if he finds it's not helping him in the polls. But Vince Foster's sister wrote a piece on the Washington Post that reminds us why this is the sort of behavior human adults should be ashamed of. The title of the piece is "Vince Foster was my brother. Donald Trump should be ashamed."