Angel's hubris played a part in how it was possible to convince him that he was destined for Twilight and his failure to consider how dreadful the plan was when it involved pulling Buffy down and making her vulnerable was bad. Warning bells should have been sounding loudly and he just
I think you are right, Angel was over confident and probably believed he knew the outcome before it even started. He had total faith that he could achieve what he'd set out to do, and a little humility would have gone a long way.
I do agree that he was manipulated, but the difference between that and being sired that we were discussing in the other thread is that he had opportunity to go away and look into it more, find out more, ask other people etc. When someone is being killed/sired by a vampire they are rarely even told what is happening from what we've seen. Certainly neither Liam nor William were given any opportunity to even hear what Dru/Darla intended for them, let alone walk away and reconsider. They didn't actually chose to go along with what was happening in any meaningful way.
I agree that Liam and William couldn't have all the facts or any comprehension of what they were letting themeselves in for. But I do feel that Angel was faced with the same issue, he couldn't comprehend the myriad ways Twilight would affect the world. He certainly didn't take into account Buffy's reaction to Twilight. I think one of the problems is that Twilight was based on some sort of prophecy and in this verse prophecies can be read a thousand different ways, and no amount of research could have covered all the possible outcomes.
I do agree though that Angel should be forgiven because I do think that, however foolish, he did believe that it was a greater good situation. But I still think of him as being culpable for getting involved in the situation. He should have questioned it because of what it involved, if for no other reason. It is somewhat different from Faith's mistakes because he was manipulated (although it has to be said the Mayor did a good job of making Faith feel separate to the others too), but his own character faults played a big part in how easy that was to do (like Faith's distrust/jealousies), so I don't think it can be seen to be totally out of his (or her!) hands, and I really don't think he (or she*) sees it like that either. I think Angel felt he needed forgiving in S9 when he was talking to Willow, even if he did still want to hold clear that he had thought it was the right thing to do at the time.
I think the differences between Angel and Faith are minimal in this situation, with Angel being maniuplated, and Faith possibly being groomed. But I feel both of their personalities were part of why they were played so easily by Twilight and The Mayor. Angel wanted to make amends for what he'd done and felt Twilight would give him that opportunity, to make a better world. And Faith wanted to be loved and respected and that's what The Mayor gave her.
(*Okay I talked myself around in a loop there, they both were manipulated and should both be forgiven as they have both seen their fault in it. 😀)
They are both forgivable in my eyes too, but I'm not Buffy and I totally understand why she finds it harder to forgive the girl who tried to kill her and her lover (Angel) and slept with her boyfriend and took over her body. Faith's attacks were personal, while Angel/Twilight's were more general, and she and Angel loved each other, while she and Faith only had bad history.