"I offer you this wisdom, Aang. Only justice will bring peace."

Context Spoilers for Centaurworld below:

Elktaur (Seen above) loved the Princess (Seen above, much older), but didn't think she could love him as his goofy Elktaur self. (She totally did though) So, he did something unthinkable. Using a magic device, he split himself into a Human and an Elk. The Human part got to run off and woo the Princess, while his Elk self was left to stare from afar, still very much in love with the Princess.

He later approached his Human self, and said that they might have made a mistake, and should rejoin, but Human him didn't want to fuck up the good thing he had going, so he imprisoned his Elk self for years in a dungeon so small he couldn't even stand up. (He tried to kill him, but would have died himself)

After the Elk managed to escape, with the help of the Princess, he became corrupted, and started a war between his Minotaurs and the two worlds, responsible for thousands of deaths and years of suffering.

This war finally comes to an end at the series finale, where the two halves rejoin on their deathbed, and the Princess delivers the final blow.

Poor guy caused a war because he couldn't get her hints.