I'm interested to note if there are similar sentiments held for anyone who was being expansionist like Napoleon, for instance. Was there a popular scapegoat dictator of sorts?
I'm not sure the premise of this question is accurate. Hitler was not/is not despised because of his expansionism or irredentism. He's despised because he oversaw a massively organized, industrial scale genocide.
On a different note, if you're asking who the go to "worst person ever" was before Hitler, this question has been asked before, and the TLDR is "it depends on where you're from". Here are a couple of times this was asked before: 1, 2
It should also be noted that even Hitler is not universally despised. While he certainly is in the West, there are places where his impact was not very great where he is seen as a sort of positive figure, what he did to Jews and other so-called sub-humans notwithstanding. A good example of this is how Hitler is viewed in India.
I think the OP's question could be clarified a bit (then maybe there'll be some on topic responses).
Today people say 'so and so is just like Hitler' or 'X policy is what Hitler did'! They do this as a means of writing them off (see Godwin's law). The words 'fascists' and 'Nazi' are often deployed in similar ways.
The OP wants to know if there have been similar persons that were the popular go to comparison for delegitimizing someone else. Specifically, if any past dictators or conquerors were used in a similar way rhetorically.