Though I don't like AC Cloud either you are reading WAAAAY to much into this.
He says he's not fit to help anyone, that there is no cure but what he says most of all is that he wants to be forgiven. That's not along the lines of what you're saying, he's contrasted with Vincent who flt out states that never tried to be forgiven. And you really should check out the Complete version.
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Marlene says that what he was doing while away actually WAS looking for a cure and he doesn't deny it. And most of all after the credits he comes to the place where Zack died with Denzel and when Denzel asks him what this place is Cloud says "This is where a hero started his journey." Referring to himself.