Evil is a BS theological notion.The origin of the term evil is theological. But the word and concept of "evil" still is useful in a secular world.
My definition of evil: "A person is evil if they repeatedly cause large amounts of unnecessary suffering to innocent people, as a result of a correctable character deficiency." If someone repeatedly causes net increases in suffering due to greed, pride, vanity, callousness, self-serving ignorance, desire for power, lust, sadism, then I think that person can be accurately characterized as evil.
Hitler was evil because he caused a tremendous amount of suffering that would not have happened otherwise, due to his greed and lust for power. Large numbers of the German population at the time were also evil because they supported Hitler out of greed or self-serving ignorance.
Harry Truman was not evil. He dropped the A-Bomb on Japan because he honestly believed that it would bring the war to a conclusion in the fastest way possible. Even if he may have been incorrect in this analysis, from my knowledge of Truman's thought process, the mistake would have been an honest, tragic mistake, not a mistake due to a severe deficiency in character. Also, Japanese civilians were not innocents, World War II was a total war in which all rules of war had been violated and all parts of Japanese society were active contributors to the machinery of war. If Truman's generals had presented alternative military options for ending the war that would not have cost vast numbers American lives, and Truman had ignored these options due to callousness to the lives of the Japanese, then I would agree that Truman was evil. But I do not think this was the case.
Stalin and Mao were evil because they created vast amounts of suffering due to a combination of pride, will to power, and willful ignorance. They had a desire to rewrite the nature of society whole-cloth and did not make the effort to discover how their experiments were actually turning out in practice.
The captain of a slave ship enslaving Africans in the 17th century was certainly evil. He was creating enormous net suffering due to his own greed. A Rhodesian settler who wiped out the natives and stole their land would also classify as evil.
Was the Governor-General of the Belgian Congo in 1955 evil? And by proxy, were the Belgian politicians the people who elected them evil? I do not think so. He did not enslave the native or wage violent war against innocents. The colonial situation was inherited from his ancestors, and from what I have read, he did his best to make the situation quite a bit better.
Andrew Jackson was evil. He stole land from the Cherokee out of greed and will to dominate, and sent them on the trail of tears.
Was George Washington evil for being a slaveholder? That's an interesting question. I would argue no. His plantation was inherited, he did not net increase the suffering from slavery via his own character flaws. Instead, his virtues helped set in motion events which would ultimately rid I might concede that he was evil relative to the average American of modern times, but that he should still be celebrated for being so much less evil than the average person of his time period.
Was the average American colonial settler evil for stealing land from the Indians? That's also tricky. The vast majority of the deaths to the Indians were due to old world disease, and cannot be reasonable blamed on settlers. At the time, the new world was incredibly underpopulated, and meanwhile the old world was quite overpopulated with most people living in poverty and deprivation. So I don't think that the settlers were evil for settling the new world. I think the situation is more tragic that the two populations could not find a way to share the land. (And contrary to the politically correct histories, this failure to share was not just the fault of the white settlers, the violence was a two-way street with fault going both ways).