A lot of the terms in your question are very general and it's not clear what you mean. Sin is pretty clear, but what do you mean by better? You need to put what kind of metric you're using to mean better. If you're just looking at straight up numbers, sure, lots of other leaders have caused the deaths of more people.

But that's not the only thing that's measured when people claim that Hitler was uniquely awful. People look at how much destruction he caused, the time frame in which he did it, Stalin for instance was in power almost 3 times longer than Hitler yet didn't create a European wide catastrophe on par with WWII. Mao was in power for even longer, if you include his leadership of the CCP during the revolution.

You can also look at who the various other contenders killed, and except for Hitler, it was mostly their political enemies, or people who were thought to be enemies. Hitler killed a lot of people just b/c they were there. There's no Maoist or Stalinest equivalent of murdering the majority of the Slavic people for lebensraum. Neither Stalin nor Mao had any philosophy that required the total eradication of groups like Jews and Roma. Stalin and Mao generally killed people b/c of what they did or might do, not b/c of who they were.

You can also look at where they killed. Most of the other contenders were killing people within their own political entities. Hitler rampaged across all of Europe and North Africa, far outside the borders of Germany and Austria.

There's also the impact on what Hitler did to his own countryman. It wasn't just the military that was out killing people. He involved the whole society in the process, from the railroad workers shipping people to camps, to people who were given stolen property of Jewish people, to every business (which was almost every manufacturing concern of any size) that used slave labor. Under Nazism, it wasn't just the military, secret police, and political operatives. It was everyone.