People always say how if they could, they would go back and time and kill Hitler before he could come into power, etc. Well, if that were to happen, just how would it affect the course of history and the present day? Would it be beneficial to humanity? Or would it have a negative impact in the long run? What key events would be impacted if there was no Adolf Hitler?
Affect. It's "affect the present day".
You might enjoy "In the Presence of Mine Enemies" by Turtledove, which tackles exactly this subject and was a very good read.
I think you would be best served by visiting /r/HistoricalWhatIf.