How did kataras mother die?

A Fire Nation soldier killed her because they were trying to destroy all the waterbenders in the Southern Water Tribe. Katara's mother told the soldier that she was the last waterbender to protect Katara. He probably burned her guts or something. Katara later found the soldier, but couldn't kill him. It was in the episode that was Zuko and Katara's adventure (that's why Toph wanted to go on a "life changing adventure" with him sometime in Book 3)