Roku, Kyoshi, and Kuruk had all tried. They had all given Aang advice for his upcoming fight with Ozai, but none of it was advice he wanted to hear.
Because they didn't understand. They couldn't understand, because they weren't Air Nomads.
But maybe Yangchen would be different. She was one of the most highly regarded Air Nomads in history. Aang had been taught many of her personal teachings by Gyatso and the other monks. If there was anyone who would understand his turmoil, it would be her. Aang closed his eyes and concentrated deep within himself to call on Yangchen's spirit.
Just as he had felt before, Aang sensed the spirit of a past Avatar leaving his body. When he opened his eyes, he saw her. She was dressed in the typical attire of an adult female Air Nomad, and the hair on the front of her head was shaved off to display her airbending master tattoos.
She spoke. "I am Avatar Yangchen, young airbender."
"Avatar Yangchen," said Aang, "the monks always taught me that all life is sacred. Even the life of the tiniest spider-fly caught in its own web."
Yangchen nodded. "Yes, all life is sacred."
Aang smiled. "I know, I'm even a vegetarian. I've always tried to solve my problems by being quick or clever. And I've only had to use violence for necessary defense. And I've certainly never used it to take a life."
"Avatar Aang, I know that you're a gentle spirit, and the monks have taught you well, but this isn't about you." Yangchen's stoic expression vanished, and was replaced with a tender, sympathetic look. "I'm familiar with the turmoil you're facing right now. How much do you know about Unanimity?"
Aang recalled reading about this when he studied Yangchen's writings. "Unanimity was the code name for a plot you stopped. Some merchants were causing terror and destruction with three combustionbenders so they could free themselves from the authority of the Four Nations. You tried keeping the three combustionbenders occupied would deter them but it only made them want more power. But you eventually stopped them."
"Yes, and it was while stopping Thapa, Yingsu, and Xiaoyun that I learned a very important lesson." She sighed. "History likes to remember me as the perfect Avatar, but that's not true. There are lots of things I've done I'm not proud of."
Aang was shocked to hear that, but considering his own experiences the shock didn't last long.
Yangchen continued. "Many great and wise Air Nomads have detached themselves from the world and achieved spiritual enlightenment, but you and I cannot. As the Avatar, your duty is to the world and the world alone. You must prioritize that duty above your own personal beliefs."
"So… I have to kill Fire Lord Ozai?" Aang looked distraught.
Yangchen's spirit moved closer to Aang and gave him a hug. "I'm so sorry, Aang. If I knew of a way to stop him non-lethally I would tell you. You must do whatever it takes to protect the world, but that doesn't make you a lesser Avatar nor a lesser Air Nomad. That's the lesson I learned from my closest friend."
"Master Kavik," said Aang.
"Yes." Yangchen released Aang from the hug and sighed deeply. "I had no idea what Unanimity even was at first, so I tasked Kavik with infiltrating the zongdus' associations. I discovered that two men were killed directly because of my actions, and another died because Kavik flooded an important warehouse. Three men died thanks to me, and that was all before we found out what Unanimity was." Yangchen decided not to mention Kavik's betrayal and atonement. It wasn't important to the current situation. "I initially managed to defeat the combustionbending trio without any casualties, but that victory didn't last forever."
"Because they got loose," said Aang.
"Unfortunately," said Yangchen. "Their destruction was terrible. The city of Bin-Er was practically destroyed by a few blasts, and they were moving on to Ba Sing Se. I had to stop them from taking any more lives, and I knew that they were too powerful to be simply subdued and captured. That didn't work the first time." She sighed again. "Kavik helped me realize that although taking a life is a vile, horrific act, there are circumstances where it must be done for the greater good."
"You… killed them." Aang never thought that the Avatar who was often seen as the perfect Air Nomad would ever kill, but apparently she did.
"Well, Kavik killed Xiaoyun but I did kill Thapa and Yingsu." Yangchen shed a few tears. "I took absolutely no pleasure in doing it, and I did it as quickly and painlessly as I could. I did it because it was my duty as the Avatar. Unfortunately, it seems that no matter how hard we try to prevent it, being responsible for someone's death is something all Avatars will have to face at some point." Her tears stopped. "The number of lives I saved that day far outweighs the two I ended. Fire Lord Ozai intends to kill everyone in the Earth Kingdom. Don't think of the single life you'll be taking, instead think of the millions of lives you'll be saving."
All of a sudden, the former Avatar started fading away into mist. "That is the wisdom I offer you today, Aang."
And then she was gone.
Aang could try speaking to Avatars further back in the cycle. To Szeto and Salai and Gun, and so on, but that wouldn't really make a difference. They'd all just reiterate the point Yangchen so clearly spelled out for him: Ozai had to die. He was still easy and unconfident about this, but he felt better than he had before he spoke with his past lives, back when he lashed out on the Ember Island beach.
Momo chittered and crawled up to Aang. The young airbender looked up toward the sky.
"I guess I don't have a choice, Momo. I have to kill the Fire Lord…"
This discussion probably doesn't change anything and Aang would still learn energybending anyway, but I think that Aang's perspective would've changed a little.
Also, I thought it was a bit odd in the actual episode that Yangchen didn't mention something from her past when the other three Avatars did, so here we are.
Yes, Yangchen and Kavik end The Dawn of Yangchen not on the greatest terms but I believe they'll reconcile in the second novel. Also, Kavik is a really good waterbender, so it's reasonable that he could become a master eventually.
And considering a major theme of the novel is Yangchen having to come to terms with the aspects of herself she doesn't like, so I think the second novel is when she will have to rationalize taking a life for the greater good, the knowledge she canonically gives Aang.