Stalin sent Ramón Mercader as a "student" who wanted to be enlightened on Communism by Trotsky to Mexico. This was part of Stalin's plan of getting Trotsky assassinated.
So, tell me more about Communism.
Following an extrended period of time, Trotsky's security stopped checking Mercador for any weapons as they became familiar with and trusted him. One day, Mercador used this advantage to bring an ice axe with him, which he used to assassinate Trotsky.
After his plan worked, Stalin was at peace knowing that he had no competition to worry about. Stalin had complete control over Russia and the Russian people.