Not to discourage other responses, but you may be interested in checking out previous threads on this question in our FAQ: https://www.reddit.com/r/AskHistorians/wiki/theory#wiki_who_was_considered_the_.22most_evil_person.22_before_hitler.3F

edit: some of those are quite old, this more recent thread is more extensive: https://www.reddit.com/r/AskHistorians/comments/41zzc6/before_hitler_and_the_nazis_was_there_another/?ref=search_posts&utm_content=comments&utm_medium=front&utm_source=reddit&utm_name=AskHistorians