It’s not very accurate to link Stalin’s negative leadership aspects to Marxism.
Marxism has nothing to do with being authoritarian , unethical or a bad character.
It’s a very valid, powerful and insightful framework or tool to understand the society and has many branches and implications.
As any analytical tool, it can be used for bad or for good, and it has its own limitations and scope of use. And because of the dialectic dynamics it acknowledges and recognises it has evolved and will continue to evolve towards more and more useful syntheses.