Why would you be charged for permanently crippling someone in a fist fight?

It's called "grievous bodily harm".  It's a crime.

You shouldn't need to defend yourself with your fists to the point of crippling someone, especially if you weren't physically attacked. Verbal attacks are not sufficient justification even for landing a punch, not under the law.

Self defence only applies as an excuse if you were being physically attacked.

Someone winding you up or generally badmouthing you won't get you off the hook if he winds up in a wheelchair.

It sound like you need some court-ordered anger management classes for those mental health issues.

Because that goes beyond any reasonable attempt to defend yourself. Unless they have a deadly weapon, and are threatening lethal force, or you reasonably feel afraid for your life, going as far as to cripple, or kill someone is going too far.  You can always walk away, or detain the person attacking you.  Crippling someone unable to defend themselves anymore makes that person in the wrong. 

Why didn't you walk away?, is what the law basically says on this. 

It's force against like force.  Deadly force, for example, is only legal against deadly force.

Was there a reason for crippling the other person?

Wouldn't it be easier to walk away?

If you can see if theirs cameras around and you cripple the other fool in self defense, you ight

If it was in self-defense, then the person who crippled the other shouldn't be charged.