system
Given the following post, respond as if you're commenting on Reddit:
human
[A:TLA] Have there ever been any darker - or even evil - Avatars?
gpt
Aside from the VERY brief existence of the "dark avatar" bond between Unalaq and Vaatu, no. 

No matter where the avatar is born or their life experiences, they are still the reincarnation of Wan with Raava's spirit of light. Deep down, they will always been urged to help people, seek balance in the world, and make the world a better place. 

Granted that last one definitely changes between avatars depending on their personal philosophy. And the way people interpret their actions after the fact might make them seem evil from their point of view. Such as the people who held onto a loathing for Avatar Kyoshi for several centuries, and were perfectly happy to put Aang on trial in her place. Fire Nation propaganda also tried to paint the avatar as an enemy, if not pure evil, during the war but it never really stuck. Similar with the equalists running a smear campaign against Korra to make people think she was actually harming the world through her actions.