Why Malcolm X so controversial?

Mostly because of his approach. Where Martin Luther King Jr. promoted peaceful protests and non-violent civil disobedience, Malcolm X believed more in fighting back to the point where he was accused of promoting black supremacy.

If you are familiar with the X-Men, it is similar to the difference between Professor X and Magneto. Professor X wishes to find a common ground with the rest of the world where everyone can coexist. Magneto prefers a world where his people are in control.