A documentary filmmaker is not just a journalist. Most of the principles for making good fiction also apply to a documentary
film. A documentary does not need to have an agenda (it is better if it doesn't), but it must have a point.
Some documentaries I have seen have been mostly a random collector of clips on a theme, and that is just not enough. There must be a thread between things that you see.
If the film presents a lead, then that lead should be followed through and not left dangling, otherwise it should not be in the film at all.
There are a few well-known documentary films about controversial subject matter, that have received good reviews and received several awards and been talked about all over as " a must see", but when I got to see them they have made me angry. Angry, not (only) for the subject matter, but I have felt cheated because they have been so bad on a cinematic level.
Make it a love letter to Hitler and I won't make it past the opening titles.
Sorry, but that sentence made me LOL. Such a documentary is just
so improbable.
I have seen some really good documentaries about the 1930's and the war from a German point of view. When I was in Ninth Grade, I wrote a long essay about Hitler and my best source was a documentary film that focused on Hitler's
career. That one was really well made, objective and interesting, even though it did not say much about the atrocities. It was implied that the viewer would already be well aware of the worst things. The point was to show how Hitler had risen to power, not what he did when he had got there.
By the way, another thing that I came across when I worked on my essay was a
children's book about Hitler 
eek !!!) that I found in a deep dark corner in an old library. It had been published in Swedish not long before the war, and I would not be surprised if it had even been produced by the Nazi propaganda machine. It presented the Hitler Jugend as a fun club to be in, things that the regime took credit for and about how Hitler liked the outdoors and how he loved his dogs.
That was indeed something that you could call "a love letter to Hitler". It was a surreal thing to come across, for sure.