I think "could X have stopped Hitler" is asking the wrong question (and it betrays the same kind of black/white thinking that is fundamental to the rhetoric I mentioned).
The Treaty of Versailles was crippling the economy, the royalists felt betrayed by the social democrats who ended the Great War by surrendering, the Great Depression destabilised the banks, and so on. Even a bullet in the brain wouldn't necessarily have "stopped Hitler" at the time (the reason we talk about "stopping Hitler" is that Hitler wasn't stopped so we don't know who'd have taken his place and where that would have led the movement at the time).
But this is irrelevant because 2010s Germans aren't 1920s/1930s Germans. West Germany underwent a massive process of Denazification, we entered into and established ourselves in the European Union and Germans who grew up in post-war Germany are used to the idea that incitement is a crime and racial hatred has no place in a civilised society.
It's irrelevant whether criminalising incitement ("censorship") would have stopped Hitler in the 1930s because a single law doesn't instantly transform society. It's 2018 and this is not a new law. But we'll never be able to tell whether it stopped Hitler today because if we stopped him we won't know who he is because he'll never ascend to the same significance in history in the first place. It's a strawman.
You also seem to be under the false presumption that you having legal "freedom of speech" means you can express anything you want without consequences. As is being pointed out all the time, freedom of speech doesn't mean freedom from consequences, just freedom from direct legal consequences.
If you hold no significant political power and say something other people find disagreeable, you will be called out at best and ostracized at worst. In fact, freedom of speech might actually help those seeking to harm you because they have the righteous fury on their side.
If you hold significant power, there are plenty of examples in US politics how that usually works out (though time will tell how it works out for Trump in particular).
Fun side effect: dehumanising people also makes it easier to justify war. The US has been in a perpetual state of war since WW2. Do you even remember how to recognise propaganda and war rhetoric if it's been part of the mainstream since before you were born?