You could have an entire evil group of waterbenders who instantly kill people by blasting all their blood from their bodies or freezing it in their veins in an instant.
Imagine the wars. People being set on fire, crushed into flat objects with earth, dissolved into sand and bone by waterbenders.
Maybe those wars did at one point happen, but it got so brutal that all sides decided to stop before they wiped themselves from existence.
But then you'd have to ask, "so there must've been a special group for each nation that exists just to safeguard all this evil shit from happening"
Of course, we're talking about zero limitations on censorship here, and worldbuilding from that presumption. And in that case it's not really Avatar, because that world was made with Nickelodeon in mind.
