Why are communist leaders considered dictators

It depends on what you mean by communists, leaders, and crime against humanity. All those are vague politically loaded terms.

Communists are just label. I can think of a few communists leader that are "fine". I called them fine because they help their countries out of communism. So in a sense they're communist leader. In a sense they're not. These people does not commit any crime against large number of humans.

In fact, the kind of communist leaders that do not commit such crime are usually the exact same guy that want to promote free market. You can define crime against humanity in pretty broad terms, mass murders, mass rape, frequent wars, censorship, trade restrictions. The more a leader support free market, the less likely they commit crimes against humanity.

Once economy moves to free market, mass murders simply stop.

Deng Xiao Ping is the guy that bring China to prosperity. No mass murder.

Gorbachov is the guy that raise white flag to the west. No mass murder.

Lee Kuan Yew often said that Singapore is a socialist country. That makes him a communist leader. However, murder rate in Singapore is lower than in US and their percapita income is 1.5 time. Perhaps Singapore didn't practice the more heinous aspect of communism, like minimum wage.

I wouldn't call Pinochet communists, however, he is a leader in a country fallen to communism. Hence he is technically a leader of a communist country that somehow manage to reform his country's backward socialist society to a liveable one for the productive. He promote free market and hence is innocent of the worst crime against humanity, namely trade restrictions.

Obama is also a communist leader. However, he hasn't done mass murder yet because his population is well armed. He tries to create gun free zones so that some psycho can kill even more people and he can then use it as excuses to disarm the population. His plan will work because the majority of his people are commies that are just eager to slaughter the productive after looting good people through welfare and other social programs.

Also there are many leaders with "communist" like goal, like helping the poor and the oppressed, that actually fight against crimes against humanity.

Milton Friedman, for example, defend the right of the poor several time when he demands elimination of racist laws like minimum wage, anti prostitution laws, and war on drugs.

I don't think Milton counts as communists, but he fight for the right of the poor so much and he's a leader so we may want to include him. He doesn't commit mass murder either.