The ball was already rolling by the time Hitler came into power. Hitler was just the guy who jumped out in front of the angry mob and yelled "Charge!" Even if Hitler never existed, that angry mob would still have been there just begging for someone to give them someone to blame for their problems and to offer a solution, and they would not have cared how awful the solution was.
If you wanted to prevent WW2 with a time machine, the thing to do would not be to murder Hitler, but to go back to the negotiations for the
Treaty of Versailles and yell at all the diplomats negotiating the treaty. Better still, go back in time and prevent WW1. That way, you prevent
two world wars, as the ending of WW1 made the start of WW2 all but inevitable.
You could, I suppose, go back in time and prevent the murder of archduke Ferdinand by those Serbian nationalist crazies, but that wouldn't be the best action either. That murder was just the spark that lit the fire. That was the act that started the ball rolling, but the network of alliances created the hill that the ball rolled down in the first place, and there were a series of heartbreaking diplomatic blunders that prevented anyone from stopping the ball before it became a world war. There are a large number of small changes in history during that series of diplomatic blunders that could have prevented the whole thing.
If you had a time machine and wanted to prevent the history we know, going after that series of diplomatic blunders would be the best way. If the diplomats succeeded in stopping the ball from rolling, then everyone might have stopped and noticed the inherent danger of that network of alliances, that the alliances created a giant hill for the ball to roll down. If you just stopped the Serbians, then the "hill" would have remained and WW1 might still have happened as a result of some other event.
But now we're in the land of wild speculation. What would actually have been the consequences of preventing WW1? While overall, I'm quite confident this would have been a good thing (or at least much less bad), I have no idea what the negative repercussions would have been. One of the effects of WW1 was to smash the power of the aristocracy. What would our history have looked like if the European aristocracy remained in power? I haven't a clue. I really don't. What would history have looked like if the Ottoman empire remained intact? I have no idea. What would history have looked like if Prussia still existed? I dunno.
History is what mathematicians would call an "unstable system." Very small changes in the input values can cause very large changes in the output values.
