Not to mention that the source for Lenin "knowing" that Stalin was "rude" was his wife, who Stalin had yelled at to stop sending Lenin newspapers in accordance with the Doctor's wishes. Lenin also wrote this at a time when his mental condition was quite aggravated.
Stalin's 1927 remarks on Lenin's words were as
follows: "It is said that in that 'will' Comrade Lenin suggested to the congress that in view of Stalin's 'rudeness' it should consider the question of putting another comrade in Stalin's place as General Secretary. That is quite true. Yes, comrades, I am rude to those who grossly and perfidiously wreck and split the Party. I have never concealed this and do not conceal it now. Perhaps some mildness is needed in the treatment of splitters, but I am a bad hand at that. At the very first meeting of the plenum of the Central Committee after the Thirteenth Congress I asked the plenum of the Central Committee to release me from my duties as General Secretary. The congress itself discussed this question. It was discussed by each delegation separately, and all the delegations unanimously, including Trotsky, Kamenev and Zinoviev,
obliged Stalin to remain at his post."
The book
Stalin's Letters to Molotov also has Stalin discussing the "will" and Trotsky's machinations in relation to it.