Historians are still unsure about this, and there is compelling evidence to suggest he wasn’t a vegetarian. But it is actually irrelevant if he was. Using this argument implies that everything that Hitler did was wrong and we must do the opposite. But this isn’t a sound argument. Hitler liked dogs, took showers, brushed his teeth, ate and slept. Should we avoid doing these things because he did? Of course not!
In addition, why should we solely focus on Hitler when talking about veganism? If we look at all dictators, murderers, serial killers, rapists, and terrorists throughout history, the vast majority are meat-eaters. So if we’re going with the argument that the diets of criminals should be avoided, why are we eating meat?