You've been mislead in a very serious way. That "warning," which is more critical of Trotsky than of Stalin, comes from Stalin's only conflict with Lenin, which happened to take place during Lenin's last days, over Stalin's supposed "brutality" in Georgia. Since Lenin only got this information second-hand (he was bedridden), he never published that "testament." Once he recovered, he began to investigate personally. After he did, Stalin was acquitted.

On another note, those "left-wing Bolsheviks" like Zinoviev and Kamenev betrayed Lenin at the very hour of the revolution. Trotsky had been against it until that hour. Stalin and Lenin had been working together to bring about the revolution for decades, longer than any of these "left-wing Bolsheviks," and on top of that he never backstabbed Lenin. The idea that Lenin and Trotsky of all people were close friends and Stalin was a distant, shifty politician lurking in the background waiting to take power is absurd. Stalin was the only legitimate successor to Lenin, especially since he built the party into power even before Lenin's death.

Stalin saved the world from the Nazis too, so you might want to show a little bit of respect for this great leader. I think part of that respect involves doing your research before you unknowingly reiterate these lies in an explicitly pro-AES community.