Flamemario12

Wait? Are you going tease the user flamemario12?

Flamemario12 is the true main villain in the avatar universe.

Season 4 (100% fanon)
In his avatar wiki life,a lot of user teased him,bully him,sent a bomb to him etc. So he went to china and kill aang. But he went to avatar universe instead.

Strike back
On the wedding where aang and katara getting married,he ruined the the wedding and kill sokka. (Who cares,he revived by katara in next episode) And summon a bunch of monster to destroy the avatar universe. Aang and his team joined force and destroy some monster.

Death
His ship was destroyed by Aang. All the user celebrate to his death.

Monster summon
Summon a bunch of monster to destroy the avatar universe.

Machine maker
How did he do that?

Enemies
As for him,he has not allies,instead,he got enemies instead.

Aang
He wants to kill aang.

Katara
Katara treated him as an enemy. She want's to kill him for destroying the avatar universe.

Avatar wikia
Didn't appeared,but he did said this: "Avatar wikia is my greatest enemy in my whole life."

Trivia

 * How could and character died so easy? Write more!