So Hitler was guilty of no crimes then? It would have been a good, or at least morally obligated thing that nobody stopped him, yes?
Contrary to what is expressed above, Hitler did much more than give speeches. He also gave orders which is a different thing entirely.
Saying "Gee I wish so-and-so were dead" is not a crime in any reasonable democracy. Telling your subordinate "I order you to go and kill so-and-so" is. Do you see the difference? The person following and the person giving the order are both responsible.
Are you arguing that the people who physically pulled the triggers or shoved prisoners into gas chamber do not bear any responsibility? At any point, all that was necessary to stop Hitler was for people to not listen to him. What would help that is providing an environment of healthy dissent, not stifling it.
I also mentioned above specifically that we should hold people accountable for the actual consequences of their speech. How you get from that to "Hitler did nothing wrong" is completely beyond me. Actual consequences mean real things that occur in reality. Not "I think something might possibly happen because of this" or "You dared to contradict an authority so you must be silenced".
The question is where the responsibility lies. Hitler did not elect himself. The responsibility for electing Hitler lies with...the people who voted for Hitler. Where else would you put it?
As to whether someone who "tries to stop him" is a bad person, that depends on what you mean by tries to stop him. One of the benefits of not censoring dissent is that we can hear many different points of view, and hopefully choose better ones. So, of course there is no problem with trying to stop anyone by making a better argument and doing a better job of rallying people to your cause.
If you mean, "Do I think it's OK to murder political candidates as long as I'm sufficiently convinced they'll be evil", no I don't. Keep in mind that there are people who feel about Obama or FDR the way you feel about Hitler and reconsider whether you want to condone murdering anyone you disagree with.