this is not so improbable
Hitler was at first a socialist, fotographs of him have been found, where he is seen attending hard line communist rallies
what is needed is that he does not get involved with any ultra-nationalist group, and develops that crazy worldview of his under the influence of Marxism
for instance he can still write Mein campf in prison, but instead of racial purity and unclean Jews he writes about some derivate of Class struggle and the hated Jewish class enemy conspiracing with other capitalist forces to controll the working class
most of the retorics can stay the same, practically all the tipical "protocols of zion" routine applies, just change the context and a few frazes
assuming Hitler at this point is the unstabile, still forming persona that he is today proposed to have been, it wouldnt take much for this to happen, perhaps an authoritarian father figure from the communist party patts him on the back a few times and makes him feel like a true comrade
really it is a question of what bar he goes to
this would be a strange version of communism do, assuming he would play a part in forming it, but it is tipical that diferent people and enviroments construct different communist practice
the problem is a communist Hitler would have much more problems getting into the Reichstag
still his ability to get crowds into orgasmic frenzy would of been a huge bonus for the German communist party, and a big problem for any anti-communist movement
could this lead to civil war?
another possibility is that Hitler, under critical communist influence, develops into a true fashist, much in the way Mussoliny started out as an actuall active socialist, and the fascio was at first the simbol of socialist republicanism, with a more statist instead of nationalist outlook