Based on stereotypes, no doubt.
why so?
I think there are a recurring stereotype of INFJs being super nice creatures incapable of doing any harm. There are people who even say that Jesus was an INFJ (the historical figure, I suppose). Mother Tereza, Gandhi, blah blah blah. As if "pure goodness" was an inherently INFJ trait.
Perhaps the reason I don't doubt the possibility of Hitler being an INFJ is a bit different from the mainstream.
Let's see: First, I used to see him as extrovert figure, but then I realized that much of his charisma and ability to persuade people could easily come from a well-developed (in a ill fashion) Fe. The Fe working together with the Ni can be a powerful weapon, I think. You can read people quite well, you can see underlying reasons and you have a good sense of what-can-be. To cut it short: You can be a massive manipulator and yet look like the most sweet person. So because of that, I think of NiFe or FeNi. I don't completely discard ENFJ but, reading about his hobbies, interests and the way historians present him, I see Hitler as much more of an introvert type. One stereotype I don't have much against is that
some INFJs may appear
somewhat extroverts, but only to a certain extent, they are still deeply introverted people, which I can see in Hitler's. That's a short explanation, of course, but I think it gives a general idea.
I can't speak for other INFJs but the ability to read into people's minds can be even a bit scary sometimes. If you have bad intentions, it certainly can get a very dark tone. That also makes me think that the award for "all time best (or worst) villains" should go to INFJ's xD.
Ni alone is very rational (for lack of a better word) and one thing that irks me is the notion that Fe is all caring and loving. It can be, of course, but it is not something intrinsic of the function. I see the NiFe pairing as a strong intuitive person who just "get" people. Be it for the good or for the bad. The Ni-Ti loop is also something worth noticing, as the person can get an INTJish manner but with some aggressiveness in it.
With all that said, there are many other stereotypes credited to INFJs that just seem like bullshit.