Here is the sentence again:
When Millie overheard Coach Jones complain that women didn't belong on a
basketball court, she became so angry that she picked up a tennis racket
and whacked him over the head, the assault
and battery charge, in Millie's opinion, was worth it.
She became so angry is a main clause. The assault and battery charge was worth it is a second main clause. These two main clauses have only a comma connecting them, causing a comma splice, a major sentence error.
To fix the problem, you could put a period after head and capitalize the T for the.