You have a time machine with a single round trip. You hate everyone. And you are lazy. What is the least you can do to completely screw over the world with your one trip through time?

I really like this question. So: rather than killing Hitler, what if, instead, you killed Stalin? Was it inevitable that a strongman dictator would have taken over, and ruined the potential of communism? I guess we have evidence that the answer is "yes," in the form of Mao, but weren't the Chinese communist party(s) greatly influenced by the Soviet model? What if Russia had, instead, developed a more democratic system of government - was it possible, and couldn't it have affected how China's developed? But, maybe it is always inevitable that dictators emerge from internal revolutions like this.

Here's another scenario: what if you stopped Oswald, and prevented Kennedy from being assassinated? He was popular, and likely to win a second term. What would 4 (~5) more years of Kennedy look like?