I have a complete opposite experience from middle school. I had a guy in my class who did some martial arts and was generally fine, but there was another kid in a different class who was practicing martial arts in the same place and was a straight bully who picked fights during breaks. And I mean like full on fights with kicking and everything, dragging other people down in mud etc.

And because that kid was studying martial arts as a hobby, they obviously had an upper hand on all the other 13 year olds and would basically just win every fight thinking they're better and "cooler" than everyone else.

So just because you're doing martial arts or have a sport to take part in doesn't make you a good person, you can still be a douche.