Ok, i’m going to spoil what the “nazi allegory thing” means, so HEAVY SPOILERS:

All those titan shifters are child soldiers sent by an imperialist country trying to conquer the island where the main characters live, also the entire world is ok with this because they are racist towards them (because they used to be a big empire that conquered and enslaved everyone and are the only ones who can turn into titans, the members of this race live in ghettos in this imperialist country and probably all the others). When the main characters find out about this, some want to leave the past behind and make peace with the world; others want to kill everyone else and commit genocide so they will finally live in peace in the island (which clearly portrays one the main characters as the main villain).