Joseph Stalin was a communist leader friend with Leon Trotsky
Trotsky was a communist revolutionary and intellectual. He once wrote "In politics, obtaining power and maintaining power justifies anything" in his book "Leur morale et la nôtre"*
In this book, Trotsky justifies the use of lies, manipulation, bribery, spying, infiltration of other political parties, even hostage taking. He says absolute ruthlesness is necessary to wield political power. He concludes "We are acting for the greater good. We can't be restrained by normal morality"
Joseph Stalin took Trotsky's advice literally. So he murdered Trotsky because he saw him as rival. Stalin also started killing people because he believed they could be sympathetic to capitalism or opponents to his personal power.
Matvei Bronstein: Theorical physicist. Pioneer of quantum gravity. Arrested, accused of fictional “terroristic” activity and shot in 1938
Lev Shubnikov: Experimental physicist. Accused on false charges. Executed
Adrian Piotrovsky: Russian dramaturge. Accused on false charges of treason. Executed.
Nikolai Bukharin: Leader of the Communist revolution. Member of the Politburo. Falsely accused of treason. Executed.
General Alexander Egorov: Marshal of the Soviet Union. Commander of the Red Army Southern Front. Member of the Central Committee of the Communist Party. Arrested, accused on false charges, executed.
General Mikhail Tukhachevsky: Supreme Marshal of the Soviet Union. Nicknamed the Red Napoleon. Arrested, accused on fake charges. Executed.
Grigory Zinoviev:: Communist intellectual. Chairman of the Communist International Movement. Member of the Soviet Politburo. Accused of treason and executed.
Even the secret police themselves were not safe:
Genrikh Yagoda : Right-hand of Joseph Stalin. Head of the NKD Secret Police. He spied on everyone and jailed thousands of innocents. Arrested and executed.
https://en.wikipedia.org/wiki/Genrikh_Yagoda
Nikolai Yezhov : Appointed head of the NKD Secret Police after the killing of Yagoda. Arrested on fake charges. Also executed.
https://en.wikipedia.org/wiki/Nikolai_Yezhov
Everybody was absolutely terrified during this period. At least 500 000 people were murdered. Over 1 million people were deported to Gulags, secret prisons in Siberia, where they worked 12 hours a day.
Joseph Stalin decided to crush Ukraine for resisting communism and supporting independance. In 1933, he seized all Ukraine's food. In the next months, 5 million Ukrainians were starved to death. The situation was so bad that thousands of Ukrainians turned to cannibalism. When Nazis invaded Ukraine, some Ukrainians thought they were saviors
https://cla.umn.edu/chgs/holocaust-genocide-education/resource-guides/holodomor
https://www.history.com/articles/ukrainian-famine-stalin
Hitler was a monster, but we really don't talk enough about how bad Stalin was.
Paulus commanded a 360.000 strong army that was previously responsible for many atrocities, including mass shootings of Jews and Ukrainians at Babi Yar - one of the cruelest scenes of the war. His capture in the rank of field marshal was a major blow to the reputation of Wehrmacht command. He made the situation even worse for Hitler by switching sides and going for the Soviets, helping the anti-Hitler propaganda machine in the German ranks.
Conventional morals may question Stalin's decision, and family values were strong in Stalin's USSR. But he was also responsible for millions of Soviet soldiers and civilians, and any blow to Wehrmacht meant closer victory and less victims. Someone else's sons and daughters.