Russia

Although Russia was an ally during WW2, they were not without their own atrocities before, during, and after, and it has been confirmed that these actions were directed by government leaders.

Beginning with the dethroning and execution of the Romanovs, followed by the White and Red Terrors, then to several famines, massacre after massacre followed one after another for decades.

As for war crimes, see Estonia, Latvia, Lithuania, Hungary, Romania, Yugoslavia, and others for examples. Rape was common, including keeping young girls prisoner, repeatedly raping them, then killing them. It is estimated that at least 1.4 million women were raped by the Soviets and this may be the greatest mass-rape in history.

High numbers of civilians were killed, and not by indiscriminate bombings which was, at the time, a normal tactic. These were round-ups and mass shootings, like the Nazis, Italians, and Japanese did.

Although the other Allied countries specifically prohibited these kinds of things from its soldiers, they turned a blind eye to what they knew the Soviets were doing since they were on the same side.

https://en.wikipedia.org/wiki/List_of_massacres_in_the_Soviet_Union

https://en.wikipedia.org/wiki/Soviet_war_crimes