system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar]What stops an avatar from abusing his abilities?

Why hasn't their been an "evil" avatar that was a tyrant? Avatars are still human after all and and humans by nature are corruptible.
gpt
Nothing.

The "rightness" of his or her actions is determined by the observer and is open to interpretation.  The Avatar is meant to preserve balance, but how they goes about that is up to them.

The best example is Aang's past lives advising him that he had basically no choice but to kill Ozai.  Aang considered that morally abhorrent because of the way he was raised.

Kyoshi is another example. She stopped what she considered a tyrant, but the tyrant's supporters and their descendants viewed what she did as an evil act.

Roku is another example, as was that water-bending Avatar got advice from toward the end.  Their inaction allowed evil to flourish and triumph.

And we also have Korra.  She was reckless and headstrong upon arrival in Republic City and caused quite a bit of damage and trouble.

We don't know the full history of the Avatars.  It could very well be some abused their power.  It may that reason is why a special society/religion was created in order to guide the Avatar through their training, to make sure they were set on the right path and didn't abuse their power.