How do I take revenge on this infamous ?

Your teacher gave you an impossible ignment. Hitler was narcissistic, nothing you say or do has any affect on a narcissistic person. No amount of proof or evidence changes them. In his mind he is right and the world is all wrong, he doesn't have a problem, the world does. In his mind he was a God, a Saint, he did no wrong, in his mind. There is no cure for narcissistic personality disorder, so even if you were a psychologist, you would have had no affect on him. Narcissistic people can spend years in therapy and have no change, because in their mind, they do no wrong, so they have nothing to be sorry for. If you went back in time and killed him, he becomes a Martyr, so I don't even think that would be revenge. They say with bullies you kill them with kindness, maybe you could have done that. Maybe you could have said something kind to him that would have made a difference in his early childhood, but you would have had to do it in early childhood. It seems like you have a tough ignment. Hitler had a wonderful strategic mind as a soldier, so it is unlikely you could have attacked him and killed him. He even had hidden runways. If you dropped a bomb on him, you might have killed other innocent people also. It is not likely as a sniper, you would have found him. I would imagine that even his food was tested for poison, so it isn't like you could have been his private chef and poisoned him. Well maybe for the sake of your story you could go with something like that. I don't know if they have Oleander in Germany.