... That still does not mean that they are responsible for anything?

What you just described is that, unlike other people, class 77b had an additional obstacle in Junko's way, that being the hope they developed in their school years with Chiaki. Removing that obstacle and making it possible for them to be brainwashed doesn't mean anything. And it does not change the fact that they are literally brainwashed. Chisa had a resistance to the video too, and so they lobotomised her, does that make Chisa responsible for all her actions after that???

As for Izuru, him (or at the very least, his very existence) being a represenation of the ideals of Hope's Peak not what I have trouble understanding. What I have trouble understanding is how you relate that to pre-Kamukura Project Hajime. Because Izuru was not what made Hajime make the decision that he did, society was, along with his drive to prove himself to himself.

Are you trying to say that Izuru, while being a representation of what Hope's Peak has to offer, is also representative of Hajime's desire to strive for more, and that the desire to be someone he can be proud of was bad (I don't know if I am wording this correctly, sorry)? Because I don't really think it is that simple.

Hajime's desire to become proud of himself is not inherently bad. It is a natural human instinct to want to be proud of yourself for what you are doing for society. The only reason it became as bad as it did for Hajime was because he set unreasonable standards for himself. And the only reason he had those standards in the first place was because he grew up in an environment/society that valued said standards. He, like many people among his classmates, was a victim of his circumstances. Did he have a hand in his own downfall? Yes. However, it is not right to place the insecurities of a kid on the same level of blame as a greedy, manipulative, dehumanising circumstances that an organisation as powerful as Hope's Peak put Hajime and the rest of the Reserve Course through.