Previous threads have dealt with an adjacent topic you may find interesting--evil in general, not authoritarianism per se, though naturally the latter comes up. Examples:
/u/kieslowskifan's answer on Napoleon (from the FAQ): https://www.reddit.com/r/AskHistorians/comments/341jam/was_napoleon_regarded_as_hitler_before_hitler/cqqqjvk/
/u/DavidlikesPeace's answer in this thread: https://www.reddit.com/r/AskHistorians/comments/41zzc6/before_hitler_and_the_nazis_was_there_another/
In the same thread /u/StudentOfMrKleks links to a number of other threads here: https://www.reddit.com/r/AskHistorians/comments/41zzc6/before_hitler_and_the_nazis_was_there_another/cz6ox5y/
These two threads give a variety of answers, mostly brief: (1) https://www.reddit.com/r/AskHistorians/comments/zv1p7/who_was_considered_the_most_evil_person_in/ and (2) https://www.reddit.com/r/AskHistorians/comments/143tb5/what_was_the_worst_thing_ever_before_hitler_and/
A recent one from /u/ManhattanThenBerlin: https://www.reddit.com/r/AskHistorians/comments/9chnrr/before_hitler/
Searching for "hitler worst" or similar terms will yield more.