Sasuke's decisions in Naruto to leave the village in pursuit of power, abandoning his friends, and betraying Konoha as a whole, fall squarely on his own shoulders. They are decisions that he makes on his own, and that he is ultimately responsible for their consequences.
But there are certainly people who contribute to the feelings Sasuke has that make him feel that this is the only way he’s going to achieve the power that he thinks he needs. Whether they actively teach him the wrong lessons about the nature of power or ignore the signs that something isn’t right, there are several people who at least claim some responsibility for what happened to him.
10 Fugaku Uchiha
Sasuke’s father is probably the very earliest person to impact Sasuke and particularly Sasuke’s view of himself negatively. He rarely gives Sasuke the time of day, obsessed with Sasuke’s older brother Itachi's accomplishments and thinking of him as the future of the Uchiha Clan. Sasuke learns from a young age that the only way he can get his father’s attention is to be even better as a ninja than his brother. This fuels his obsession with power from a very early age as his father doles out praise only when Sasuke proves himself to be a strong Uchiha.
9 Itachi Uchiha
Where Fugaku specifically urges Sasuke to seek out power by constantly comparing him to Itachi and what he’s managed to accomplish, his brother himself actually urges Sasuke to become stronger. After decimating the Uchiha clan, he leaves Sasuke alive to foster resentment and hatred in him, hoping that Sasuke will become powerful for the sake of revenge and pursue him to kill him. Sasuke specifically says that his reason for gaining power is to be someday able to kill Itachi.
8 Kabuto
While Sasuke doesn’t have a lot of direct interaction with Kabuto, outside of when he helps Team 7 get through the Chunin Exams, Kabuto’s machinations definitely help lead Sasuke toward Orochimaru.
Kabuto’s direction shows Sasuke just how powerful Naruto has gotten as he uses Shadow Clone after Shadow Clone to get them through a tricky situation. He helps Orochimaru set the stage to lure Sasuke into their arms, and he does so successfully, even when it’s clear he doesn’t want him to come.
7 Sakura Haruno
Sakura is an interesting person when it comes to the question of her responsibility in Sasuke’s downfall. Sakura and Naruto are the only reasons Sasuke has to stay. But Sasuke believes that having connections like those with Naruto and Sakura actually makes him weaker and keeps him from becoming as strong as he can. Sakura’s love for Sasuke might have kept him in Konoha if it had come slightly earlier, but she isn’t willing to acknowledge that Sasuke might be in trouble until it’s too late to save him.
6 Haku
Haku is the earliest opponent that Sasuke faces. He and Naruto fight him together, and it seems like they have met their match. Sasuke sacrifices himself to save Naruto’s life, though he doesn’t die, and he wakes to find that Haku has been defeated and that Naruto played a big role in that defeat.
It’s the first inkling for fans that Sasuke may not be quite as strong as everyone thinks (and that Naruto might be stronger than expected), and it’s also likely the first time Sasuke might feel inadequate.
5 Rock Lee
Rock Lee has less claim to blame in this situation than most people here. But he does have an early effect on Sasuke. Rock Lee challenges Sasuke to a fight, and even with his Sharingan, he still can’t keep up with Lee, even with the ability to see his movements before he makes them. It’s the first moment of Sasuke beginning to feel as if he might not be the most powerful of the Konoha 11, and the feelings of inadequacy among his classmates begin to rise at this point.
4 Kakashi Hatake
Kakashi is perhaps one of the characters most at fault in that he must see what is happening to Sasuke and fails to step in to try to change Sasuke’s course until it’s much too late. Considering Kakashi’s experience with Obito and his understanding of the Uchiha curse, it should definitely have occurred that perhaps Sasuke needs more support. Teaching him Chidori is Kakashi’s way of helping Sasuke get stronger, but in the end, he uses this jutsu to try to kill Naruto, which Kakashi might have expected.
3 Orochimaru
Sasuke might have someday sought unsavory means to achieve power even without Orochimaru, but the introduction of the two characters changes the course of Sasuke’s life. Not being able to defeat Orochimaru and having Naruto step in to rescue him puts the first inklings into Sasuke’s mind that perhaps there’s another way toward the power he needs. Orochimaru giving him the curse mark that Sasuke finds gives him the power and lack of conscience he needs to win several battles, only further convincing Sasuke that Orochimaru is the right path.
2 Naruto Uzumaki
Naruto is one of those characters who probably should have recognized that something was going wrong with Sasuke. Naruto is often seen in the series talking characters off a ledge, convincing them that they’re on the wrong path and to try to course-correct. He fails to do this with Sasuke. What’s more, Sasuke’s insecurities about Naruto becoming stronger than him are the inciting incident that leads to him seeking out and following the Sound Four.
1 Gaara
Sasuke is out of commission for a long time after the second test in the Chunin Exam. When he returns, it’s to fight Gaara in the finals, but they never get to actually finish this battle in the arena, as the Sand and Sound Ninja launch an attack against the village. Sasuke pursues Gaara when he runs away and is ultimately completely unable to defeat him. Watching Naruto protect him and Sakura from Gaara and successfully take him down drills down into Sasuke’s sense of self-worth.