They are being hyperbolic.
- In reality Stalin was no worse than Churchill.
- Stalin was unable to be beaten by the U.S. So like any other communist he has to be defeated by other means—propaganda.  Mass murderer, pervert, pedophile, racist or homophobe—just pick one or several which the U.S. propagandists will attach to an effective communist.  Every year Mao Zedong manages to kill another 10 million people.     
- Stalin did some wrong things. As a committed Stalinist I am not comfortable saying “Stalin did nothing wrong.” But sometimes I feel like saying it to the indoctrinated toadies which see fit to make me known for the first time that Stalin was a “mass murderer, homophobe, or despot.”  As if I have never heard that before.  
- I have thoroughly researched Stalin and the USSR. And no, I did not rely on Dima Vorobiev’s propaganda and only on Soviet official statistics. Anyone who troubled themselves to check my sources (which I always cite) would know this. I rely upon a wide variety of sources, primarily reliable Western historians. And no, anecdotal evidence from “escaped” former Soviet citizens is not historical research.
- In this answer I cite my own answers. Within each of these answers there are citations to reliable sources. So if you happen to be especially lazy and claim that I am “citing myself” you would be wrong.
- Alexander Finnegan’s answer to Did the ends justify the means in the US alliance with the USSR during WWII? After all, it is stated that Stalin killed over 90 million of his own people.
 Let’s Be Honest – Stalin Was Less of a Criminal Than Churchill, Truman, and LBJ
 Alexander Finnegan’s answer to Was Joseph Stalin a successful leader or an oppressive dictator?
 Alexander Finnegan’s answer to Is American Media more propaganda than truth?
 Alexander Finnegan’s answer to Were there any logical reasons for the Stalin’s Great Purge or he was just paranoid?
 Alexander Finnegan’s answer to What do you think about the politics and actions of Che Guevara?
 Alexander Finnegan’s answer to How true is the claim that most Cold War propaganda about the Soviet Union is regurgitated directly from Nazi propaganda?
 How Mao Greatly Strengthened China
 Monthly Review | Did Mao Really Kill Millions in the Great Leap Forward?
 Godfree Roberts’s answer to Was the Chinese cultural revolution under Mao absolutely necessary?
 Chuck Garen’s answer to Was Holodomor a Genocide or a crime against humanity?
 Gwydion Madawc Williams’s answer to Did the Great Leap Forward really kill millions of people, or is it just propaganda?
 Alexander Finnegan’s answer to What were some of Mao’s best ideas?
 Alexander Finnegan’s answer to Should Mao Zedong’s portrait be retired and removed from Tiananmen?
 Alexander Finnegan’s answer to When did Western governments and the public learn of Stalin’s atrocities?
 Alexander Finnegan’s answer to What is the most biased book you’ve ever read?
 Alexander Finnegan’s answer to Why does history remain hard to grasp with some people?
 Alexander Finnegan’s answer to Does Alexander Finnegan think the USSR had no problems or that the standard of living was superior to American living standards?
 Alexander Finnegan’s answer to Has Russia been able to fully pull away from communism?