I'm really glad I took karate lessons as a kid.
Is there something about karate that you are proud of?
Because I can use my knowledge to defend myself against attacks
It's good to learn things and participate in sports as a kid.  Martial arts is a healthy hobby, and one that builds responsibility and discipline.  Just make sure that you are not overly confident in your abilities, though, as fighting is dangerous even for those with skills.
I will show you how dangerous I am. I want a fool to challenge me.
You shouldn't do that it is unwise to use your skills against someone innocent. It is great to learn new things as a child and participate in sports. To be able to defend yourself is good especially when you are alone at night.