If Mao Zedong killed 40-million. True. How come his picture is still in Beijing china and on the currency?

As others have said he is a big part of Chinese history. When I lived in China I asked a lot of people about Mao and most really don't know much they worship him and don't know really why. He did many bad things and some good. But why he is as important today Is hard for me to understand.