Before Hitler became the comparison for anything that was extreme bad or evil by a ruler, who was it before Hitler and why? When was the whole world pretty much like screw this guy? or gal?

In large parts of Europe, Napoleon.

Today we mostly glorify Napoleon as a great man of history. Not always in the positive sense, but we focus on his personality and historical impact.

Less mentioned is the sheer amount of destruction inflicted by his wars. It was "the war" untill WWI came along.

Napoleon was a popular choice in a lot of places.

In Europe it'd be mythical Bible villains, especially the Egyptian Pharaoh in Exodus where god interfered with his supposed free will to not free slaves that they never had in reality. Or earlier historical figures like Attila the Hun, Genghis Khan, Ivan the Terrible, Napoleon Bonaparte.

Ramses II was a common one due to the bible, as was Judas and I think Brutus and Cassius for betraying Caesar. The Divine comedy features the latter three being chewed on by Satan in the deepest pit of hell.

The bishop of Bath and Wells

Gengis Khan maybe?

I can’t help but think he left more than a few people upset. At least any that somehow escaped his reign of massacre.

That’s by global impact, not necessarily notoriety in early western civilization. He hasn’t been around few a few years by my limited understanding.