What was Lenin's main concern about Stalin?

Answer 1

Lenin felt that Stalin was far too harsh and that he would not make a good leader, but after his unexpected death in 1924, Lenin did not name his succesor, and Stalin took that chance to take power.

Answer 2

He wasn't afraid of Stalin. Lenin could have easily had Stalin executed anytime from 1917 through his death. Lenin believed, according to his testament, that Stalin would destroy the "world revolution" and believed that if given the power Lenin held during his tenure, Stalin would abuse it. The Central Committee ignored Lenin's testament, and by 1938, the majority of the members of the Central Committee deeply regretted doing so.

Lenin found Stalin too power-hungry and greedy to be the ruler of the largest country in the world, but never stated he feared Stalin.

Answer 3

Lenin was the mentor of Stalin however, overtime Lenin started to not trust Stalin anymore. Before Lenin died he made sure that Stalin didn't take over as the commisar (highest position). Many communist leaders believe that Stalin was paranoid. This paranoia would lead Stalin to killing over 23 million of his own people.