The loss of a loved one 

In the begging of the series we meet Katra and Saka, a brother and sister from the Southern water tribe. Their home was invaded by the fire nation in a tempt to capture all the water benders from their tribe. Their mother was corned by one of a fire nation's guards and was sadly captured and killed. This led Katra to take on the role as a mother to her brother and her friends throughout the series. It is well known the close bond that was shared between Katra and her mother. Her passing caused Katra to build a strong hatred towards the fire nation entirely. Throughout the series Katra shares her tragic experience with the fire nation to point out how that entire nation has little to no regard for innocent lives out side of the fire nation.