First of all I would stop the discovery of America by Columbus as it has caused the total annihilation of the native people. However, if I can not stop this momentous event I would like to stop the Slavery of mankind by other cruel human beings. I would also like to provide a human angle to the process of scientific developments.
I would probably try and stop the use of cars, i say that because if we have less cars invented then their would be less pollution, pollution hurts us humans and it puts more poisonous gasses into the air and harms multiple organisms. . . . . . . .
Everything happens for a reason. If you were to stop it from happening, the universe will correct it by finding another way to make that event happen. There is no way to prevent it, nature must take it's course. It does not matter what you do, the universe will always find a way to do what it wants because whatever happens is destiny. It is supposed to happen so it will.
According to Murphy's law, If you perceive that there are four possible ways in which something can go wrong, and circumvent these, then a fifth way, unprepared for, will promptly develop.
Corollary: It will be impossible to fix the fifth fault, without breaking the fix on one or more of the others,
I would abolish liberals. They are the scum of the earth. They are the weakness of the human race. Without them, we are strong. With them, we are a bunch of pathetic crybabies. I would stop the Democratic party from forming. Also, I would destroy Huffington Post and BuzzFeed, the most horrible of all liberal offenses.