"Evil" is a concept that can defined in a number of ways, from theological (evil as discussed in the Bible) or to describe acts so atrocious that "evil" seems an appropriate description. For purposes of discussion, this answer will focus on the latter use of the word.
It is hard to argue that Stalin and Hitler were not evil. Stalin ruled the Soviet Union for roughly 28 years. The exact figure is difficult to calculate because, between the 1924 death of Lenin, founder of the Soviet state, and Stalin's consolidation of total power, several years later, he wielded tremendous influence inside the Kremlin. Once his position was consolidated, which involved the murder of all potential rivals for power, Stalin carried out policies that resulted in millions of death either directly through mass executions or indirectly through imprisonment under brutal conditions and polices of forced collectivization that similarly resulted in many deaths. [Historians like Robert Conquest and retired Russian Army General Dmitri Volkogonov put the number of dead directly or indirectly attributable to Stalin's policies at 20 million.]
Similarly, Adoph Hitler can fairly be categorized as "evil" in that he advocated and oversaw the systematic murder of six million European Jews and millions more homosexuals, mentally handicapped Germans, Romas (Gypsies), and others. Hitler initiated a war that devastated much of Europe. That his actions were premeditated can be surmised by reading the book he authored while imprisoned for attempting a coup against the German government. In that book, "Mein Kampf" ("My Struggle"), Hitler was very explicit in why he believed Jews were a dangerous, threatening presence in Germany and Europe, and why they needed to be dealt with harshly.
Notions that the kinds of evil perpetrated by Stalin and Hitler had seen their end of days were crushed when a communist insurgency toppled the government of Cambodia in 1975. That insurgency was inspired by a political movement led by Pol Pot and called "Khmer Rouge." By the time the Khmer Rouge were driven from power by the invading Vietnamese, some two million Cambodians had been killed as a direct result of Pol Pot's policies, most tortured and worked to death, others executed for appearing to have been educated and, hence, posing a threat to the revolution. Khmer Rouge policies involved driving entire populations out of Cambodia's cities and forcing the people to work the fields in ill-conceived agricultural policies grounded in the notion that history had begun anew and that the Cambodian revolution began Year Zero.
In 1994, the sudden death of the president of Rwanda in a plane crash precipitated yet another instance of 20th Century genocide when the majority Hutu tribe began massacring the minority Tutsi, who had governed the country until then. Hundreds of thousands of Tutsi and moderate Hutu died. Whether this constituted "evil" is open for discussion, but some have no problem suggesting Hutu leaders who called for attacks on Tutsis were evil.
Some sources, in addition to the linkages below:
Robert Conquest, "The Great Terror"
Adoph Hitler, "Mein Kampf"
Nawuth Keat and Martha Kendall, "Alive in the Killing Fields: Surviving the Khmer Rouge Genocide"
Philip Gourevitch, "We wish to Inform You That Tomorrow You Will Be Killed With Your Families: Stories from Rwanda"
Alan Bullock, "Hitler and Stalin: Parallel Lives"
My contribution would be to challenge the premise: How are we defining “evil” – arbitrary executions, attempted genocide, gulags, -- how? To what degree is the term subjective, and what are some examples of Western cultures that have been accepted but could be seen as “evil”? I am thinking of slavery, colonialism, and especially religious persecutions such as the Inquisition. What America did to those of Japanese descent during WWII (indeed, the atomic bombs on Hiroshima and Nagasaki) could all be viewed as evils committed by a political power. I’m not suggesting that Hitler and Stalin weren’t evil; I’m just saying that your essay should include a paragraph on definition, and at least a discussion of these other “candidates.”