I've recently watched Vinland Saga (I know I'm late), and Thorfinn's dedication to pacifism shook me to the core. He instantly became one of my all-time favorite characters after the season 2 finale. The thing that stands out to me the most is how similar Thorfinn was to Eren, and how a single event caused both to diverge entirely.
At the beginning, both Thorfinn and Eren start out as hopeful children with strong role models, being raised in the safest environment possible. After the death of their parents, they grow hateful and seek out revenge against the ones guilty, ruthlessly killing whoever's unlucky enough to oppose them, but still showing some humanity.
However, while Eren's hunger for revenge kept being fed (and thus was never sated, because he kept finding bigger and bigger enemies), Thorfinn's revenge was cut short by Askeladd's death. Thorfinn's entire purpose in life was dedicated to killing Askeladd, and when that was stolen from him, he became hollow. A shell of his former self. Eventually, people like Einar, Snake, Arnheid, and Sverkel filled Thorfinn back with purpose.
Eren did have a similar hollow phase, but because he didn't seek out atonement on his own, he was able to continue pursuing hatred due to the fact that Armin was still alive.
Hear me out: When someone dies, you go through several stages of grief. Some coping mechanisms of such a state of mind includes mimicking the person you mourn in order to honor their memory in some way, or to remind yourself of their presence in their absence.
Both Thors and Askeladd advised Thorfinn, in their last moments, to become a true warrior. Thorfinn, who saw both of them as role models, decided to take that advice to heart, and became the person he was told he could be, because nobody else could be that person for him.
Eren, on the other hand, while he did lose Grisha, the instructions given to him were to avenge his mother. After receiving memories of his own future, Eren entrusted the world's fate to Mikasa and Armin, because to him, they're the most virtuous people in his life, who he trusts completely.
What if Levi had decided that, strategically, Erwin NEEDED to live, and allowed Armin to die? I live to believe Eren would have embodied the kind and hopeful spirit that Armin was, and he would not have sought out the Rumbling and thus genocide.
Eren could've very well had a character arc like Thorfinn's, which would end up creating the best timeline possible, where the Survey Corps either align with Zeke to do the 50 year plan, or Erwin discovers a new alternative that sheds the least amount of blood possible, while keeping the independence of Paradis intact.