Thorfinn is the main protagonist of Vinland Saga. In the first season of the anime, Thorfinn's character went through many events that ended up changing his perspective on the world around him.
The shift in Thorfinn's outlook was extremely important to the story. Thorfinn's development didn't stop after the first season; in fact, it continued in the subsequent season. Thorfinn's character has been far from perfect, and he has made many mistakes on his journey. But some of them turned out to be his biggest failures and continued to haunt him for years.
5 Being A Liability For Thors
When Floki found out that Thors was still alive, he became nervous, as he didn't want to lose his place as the commander of the Jomsvikings. So he planned to get rid of Thors once and for all. Floki asked Thors to join the war if he wanted to keep the village safe, and the latter had no option but to agree.
Thors set out with a few other men from the village, but his boat was stopped by Askeladd. Soon, it became clear that Askeladd wanted to kill Thors, but he stood no chance against the legendary warrior. However, Askeladd got a lucky break as Bjorn managed to get his hands on Thorfinn. Naturally, Thors couldn't let his son die, so he laid down his arms. Without wasting any time, Askeladd ordered his soldiers to kill Thors. While it is unfair to label this as a failure, it is still a significant event in Thorfinn's life that was brought about by his decision to go with his father.
4 Continuously Doing Askeladd's Bidding
After seeing his father die in front of his eyes, Thorfinn fell into a daze. He was overcome with sadness as he remembered his fault in Thors' death. Thorfinn wasn't willing to go back to his village because he didn't know how he would face his mother and, above all, how he would be able to live with himself.
When Thorfinn killed a man for the first time, he was appalled. It wasn't an easy decision for him, but it was necessary in order for him to survive. After his first kill, Thorfinn went down a path of complete destruction. He stayed with Askeladd for many years, and he continued to carry out missions on his behalf. Given that Askeladd murdered his father, Thorfinn's inability to say no and assist his worst enemy is definitely one of his biggest failures.
3 Not Being Able To Finish His Enemy
Thorfinn spent the majority of his life working for his worst enemy. One might overlook this blunder as he wanted an opportunity to kill Askeladd. Thorfinn risked his life and went on dangerous missions by himself, and it was all for nothing.
Every time the two fought, Askeladd came out on top. At no point did it appear that Thorfinn would win against his nemesis. This cycle continued for a long time until Askeladd's death. Thorfinn's reaction to Askeladd's death was also quite odd, and it felt like he was sad to see the latter die. Although it is more reasonable to assume that Thorfinn was just sad because his dream was stolen.
2 Not Heeding To His Father's Advice
Thors was undoubtedly the strongest character in the series. His tales of bravado made him one of the most well-known figures in King Sweyn's army. While his combat skills were ridiculous, what truly made him an amazing character was his way of thinking.
Prior to the start of the story, Thors was a fearsome warrior. If he had continued down the same path, he would have obtained a high rank within the army, but after he had an epiphany, everything changed. Thors faked his death and ran away with his wife. He was happy with his family, and that's all he cared about. When Thorfinn learned about Thors' mission, he started to go through his father's stuff and found a knife. Luckily, Thors stopped him and gave him a profound speech. Thors told his son, "You don't have any enemies. No one has any enemies. There's no one who's okay to hurt." Thorfinn paid no heed to his father's advice, and it really proved to be a big mistake.
1 Focused On Vengeance
As previously stated, Thors' death was quite monumental. It affected the entire cast of characters, including Thorfinn. It left the young boy with only one thing on his mind— vengeance. After seeing his father mercilessly killed, any child would become enraged, but only a few would have gone down the same path as Thorfinn.
He tried to fight Askeladd multiple times, but he always lost. In order to win an opportunity to challenge his worst enemy, Thorfinn killed and killed. He slew many people, and he didn't show any kind of remorse for his actions. It was because he was solely focused on killing Askekadd that he even forgot about his mother and sister, who were waiting for him. Thorfinn lost sight of what it meant to be human, and to a certain extent, he forgot about his own father. Thorfinn was devastated at the end of the first season when Askeladd was killed. Due to casting aside all his principles and goals, Thorfinn had merely become a killing machine who hadn't even thought about his future.