Avatar Roku said his biggest failure was not dealing with Sozin when he had the chance. But knowing what we know now, I can say with 200 years worth of foresight that Roku was wrong. Again.
Avatar Roku’s greatest failure was not working with Sozin. A failure Aang fixed 100 years later, not by beating Ozai but by working with Fire Lord Zuko and creating republic city, a dream Sozin had for the world 200 years ago.
First look at the parallels.
Fire Lord Sozin, 200+ years ago had a problem. The fire nation had evolved past being badass version of the air nomads and fully into a industrial and imperial economic power house. Prosperity never seen before brought on a population boom, massive changes in quality of living and a new era of peace and prosperity. Sozin saw all this and thought “why only the fire nation has to live this good.”
Keep in mind that Sozin was no more of a bad guy than season 1 zuko and even then his motivation taken at face value is far more nobler than zuko’s quest to restore his honor. It was Utopian, a utopia rejected by Roku out of principle and tradition.
Fast forward 200 years later. War has left many areas of the Earth kingdom colonized by the fire nation. At first Fire Lord Zuko declared his intentions to restore the traditional model of the world, completing his promise to the avatar. Unfortunately things weren’t that simple. Under colonization the combined earth kingdom and fire nation citizens actually prospered , just as Sozin predicted they would 100 years ago. Zuko faced with this new “society” chose to preserve it and ultimately asked the avatar to agree. Aang accepted Zuko’s vision and Republic City stands as a beacon of light and true unity for the 4 nations (technically 5 but meh).
Roku in his failings to trust a friend prevented Republic City from being built 200 years earlier, denying the world what would have been 200 years of technology and societal progress.
But to understand how deep this failing goes there is even more parallels to follow.
Roku and Sozin were friends, close friends. Roku said some friendships transcends life times. Roku should have known Sozin and trusted his intention. He should have saw past the dogma and saw the fire nation as more than an imperial force waiting to happen.
Now lets take Aang and Zuko 100 years later. Zuko did bad near ruthless and reckless things to obtain the avatar. Despite that Aang recognized back in season 1 that Zuko was not really a bad guy and they could have been friends a 100 years ago.
Now wait. When Aang said this, we thought he was talking about his fire nation buddy he had 100 years ago. It was almost a generality for “the fire nation can’t be that bad”. Fast forward to when Roku showed Aang that he was good friends with a happy and playful Prince Sozin I don’t think anyone made this critical connection. When Aang said “he and zuko could have been friends in another time” the authors were talking about Roku and Sozin and how their friendship transcended time and even nationalities (just as Aangs relationship with Monk Gyatso transcended Gyatso relationship with Roku).
Can it be thus easily said that the relationship and partnership between Avatar Aang and Firelord Zuko should have also been what have happened between Avatar Roku and Firelord Sozin 100 years ago?
Now look at Sozin and Zuko.
Sozin was a good man and a good firelord. He inherited and over saw the fire nations industrial age. He was an idealist that believed in spreading the wealth around. Though incapable of seeing the perceived fault in his actions he thought it was his and his nations destiny to lead the world to the next age. When Avatar Roku, his friend rejected him not once but twice, the second time leading to a pride crushing bitter defeat, Sozin went down a dark path. Having been forsaken by his brother and de-fanged by the Avatar, Sozin had 25 years to harbor darker and darker feelings. Which all came to a head when given a choice between saving a friends life and for filling his dream…he chose his dream. He spent the last years of his life being consumed by the search for the avatar, even wiping out the air nomads to ensure the cycle is broken.
Zuko was rejected by his father and humiliated before his entire nation. He was a good kid but ultimately felt forced to do very wrong things in pursuit of his dream. He was nearly consumed by his search and later the promise of capture of the avatar. He even sacrifice the hope of removing his scar in order to fight Katara and the Aang in hopes of finally pleasing his father.
Roku found contempt for Sozins ideals, choosing his duty as the Avatar over his own brother which ultimately cost him his life, his brother, and his legacy as an Avatar. If Roku had stuck by Sozin, taking his fire nation point of view and using his experience traveling the world and being a part of every people group in the avatar world to refine Sozin’s ideals…who knows what the avatar world would look like 200 years later in Korra’s time.