There is a novel by Éric-Emmanuel Schmitt that includes a fictional version of history where Hitler is accepted to art school and becomes a good person.

I'm honesty not so sure about this interpretation. It relies too much on his word, on his autobiography. Instead I think it was one of several factors. Some others he also describes in said book like a very nationalist teacher.

He describes his radicalization and hatred against Jews, social democrats, unions, Slavs, ... in a far too similar manner to be authentic. It looks more like an uninspired author reusing the same story line (being curious and sympathetic at first, after meeting them piece by piece realizing they are evil).

My interpretation is that his radicalization went over a longer period and the main point was by writing it down, not at the point of his life he's describing.

One thing he also writes is that he would have been a good architect rather than an artist but his math grades weren't good enough. This would explain all his obsession with enormous buildings and cities (I still take it with a grain of salt). That's a moral we barely talk about: learn your math so you can be what ever. Also you're never too old to learn new tricks.