I didn't say that. Anyone can learn the moves and processes of martial arts, regardless of their temperament. The key to learning is only dedication to training. A 'bad person' can learn to be quite devastating as a physical martial artist, they can smash bricks and hurt people and generally be a bad actor all around.
But this is surface. The core of martial arts has nothing to do with the belt one wears around their waist. It has to do with the content of their character. Martial arts contains within it a doorway to a deeper understanding, and this door cannot be opened by anyone who harbours bad intent. One might say that to pass through this doorway, one must be spotless in character, or perhaps passing through this doorway purifies the character (chicken/egg question).
To those of ill will, the doorway not only cannot be opened, they generally cannot even perceive that it exists. They are content to believe that they have penetrated the mysteries, risen to the highest echelon, and some cannot be persuaded otherwise.
High rank means little in the deeper sense.
In the Japanese concept of 'do' or 'way', (karate-do for example), the path one is on is a path towards improvement of character. It necessarily involves punching and kicking and so on - that is the karate part. The 'do' part is the part that looks inward, seeking constant improvement.
This is why in Japan, flower arrangers (kado) , tea makers (chado), calligraphers (shodo), and other disciplines are also 'do'. They are of course concerned with their conception of proper flower arranging, tea making, or character-drawing, but the deeper 'do' is very much concerned with the improvement of the character of the person doing it, as karatedo is as well.
When I say a bully cannot advance in karate(do) this is what I mean. And when it comes back to the physical plane, no matter if a bully prevails or not; they have still lost everything that matters. They merely disgrace themselves, the belt they wear, and the art they represent.
EDIT: Let me introduce you to kyodo, an art which I admire but do not (currently) practice:
Kyūdō - Wikipedia, the free encyclopedia
"According to the Nippon Kyudo Federation the supreme goal of kyudo is the state of
shin-zen-bi, roughly "truth-goodness-beauty",
[6] which can be approximated as: when archers shoot correctly (i.e. truthfully) with virtuous spirit and attitude toward all persons and all things which relate to kyudo (i.e. with goodness), beautiful shooting is realised naturally."
In other words, kyodo is archery. And yet, it is not, because they believe that the external component (hitting the target with an arrow) can only be achieved when the internal component is correctly performed (virtuous spirit and attitude).
It may sound mystical, but it is not. It is simply recognizing that the world is more connected than some wish to believe.