In the scope of human history, there have been countless world leaders driving war, profiting off poverty, slaughtering innocent civilians, and poisoning minds with racism, homophobia, sexism, religious persecution, and widespread bigotry. While this question itself is highly subjective and most likely will never definitively be answered, I will examine several possible arguments.
There is, of course, a strong argument that Adolf Hitler (leader of the Nazi Party and Führer of Germany for nine years) is one of the most evil humans to ever live. However, in terms of direct action, we might consider Heinrich Himmler--a member of the Nazi Party and the man who organized and oversaw the Nazi concentration camps under the command of Hitler--as more intimately involved in one of the most horrific genocides of the past century.
Himmler worked long and hard on the behalf of the Nazi Party long before his most infamous acts of destruction would take place. He was a worshipper of the occult and of Hitler himself, and elected to set up various departments of the SS to promote his anti-semitic beliefs and devotion to the "superior" Aryan race, including the SS Race and Settlement Main Office.
In 1933, Himmler, with the help of Theodor Eicke, created a model for the concentration camps that would spread across Europe. It is said that he is directly responsible for six million deaths as a result of these camps and indirectly responsible for forty-six million deaths due to his involvement in World War II.
Beyond these deaths, Himmler is also commonly associated with the heinous acts of torture and perversion committed during this time. There are unconfirmed rumors that Himmler sanctioned the production of soap made from the body fat of the victims of the Holocaust and that he decorated his home with furniture made from the skin and bones of the deceased. While we may never know whether these accusations are true, it is confirmed that Himmler authorized sterilization and medical experiments to be carried out on the prisoners of these camps. One of the more disturbing experiments involved determining the length of time needed to kill a human being through lowered body temperature; this was evaluated by either forcing camp victims to stand outside naked until they froze to death or by placing them in vats of ice water. Himmler's depravity clearly knew no bounds.
Perhaps the evil nature of Himmler is best summed up in his own words: "The best political weapon is the weapon of terror. Cruelty commands respect. Men may hate us. But, we don't ask for their love; only for their fear." He did, indeed, command the fear of a continent through the mass destruction and violation of human life. It is for this cruelty and dismissal of humanity that I consider him to be so evil.