As Benjamin Franklin wrote in 1789, nothing is certain but death and taxes. Were Franklin alive today, I suspect he'd add another certainty. Whenever you get two groups of people with opposing views, it is certain that someone in one of the groups will eventually compare someone in the other group to Hitler.
We see this phenomenon regularly in politics. For some, President Obama is Hitler; for others, Republicans are Nazis. In addition, we know that many Christians are fond of making up stories about Hitler being an atheist in spite of the clear historical record of his Christianity. This sort of over-the-top name-calling resolves nothing and leads instead to increasing polarization and hatred. The politician with whom you disagree is not Hitler. The atheist down the street? Also not Hitler.
It would take an awful lot for comparisons with Hitler to be even close to being warranted. When someone with an approving audience begins calling for an entire group of people to be placed in camps where they can "die out," one may feel at least somewhat justified in reminding people of Hitler. Even then, that doesn't mean that someone doing this is just like Hitler. It means that they are treading down an extremely dangerous path that is in some ways reminiscent of Hitler. He might raise the spectre of Hitler in such a circumstance as sort of an alarm.
But really, how many people today are standing up in front of a receptive crowd and calling for an entire group to be placed in camps? Virtually nobody, right? Here is Pastor Charles Worley giving a sermon to the congregation of Providence Road Baptist Church in North Carolina on May 13, 2012:
To the degree that Pastor Worley's views are accepted by his audience, we should be concerned. To the degree that Pastor Worley's views are echoed in other churches, we should be very concerned. When informed observers point to Pastor Worley and caution, "This is how it begins," we should listen to them.
H/T to Religion Poisons