it wasn't a rule in the show that what Giles did was wrong, it was his opinion that what he did was wrong.
& The Angel thing, yes he was innocent, but no it wasn't wrong, Buffy did the right thing, the heroic thing, if what she did to Angel was wrong, she isn't a hero. she killed Angel for the greater good.
later she came with the same choice, she told Giles that she did it once, but she is not gonna do it again, so if she didn't let Dawn jump she'd be a hero? she only redeemed herself after she actually jumped.
Twilight is manipulative, a cowardily murder, kills for the big picture, deceitful, controlling, could be seen as visionary, all of which are characteristics of Angel.
none of these are characteristics of Angel, if that was Angel, Lilah & Lindsay would be dead by the end of the first season.
Angel did it once to Lindsay, I don't know the exact reason, whether its his not-so-bright future, or that he doesn't trust him to change, but doing it once doesn't make it a characteristic.
Angel & Buffy both never believed in killing for the greater good, they did it once or twice, but they don't believe in it.
Twilight's actions are far from Angel's ways, the only way Angel would do that is if the universe unbalance by the slayers caused Connor to die. but Angel never walked around killing people for the greater picture.
Giles had a choice at the end of Season 5:
1- Kill Ben: save the lives of others
2- Let him Live: Glory would 100% at least consume brains, if not try to find another way to her dimension.
no matter what he does, a portion of people would see it as wrong, both actions are wrong, maybe no matter what he does both actions are unheroic, but to protect the lives of others, he has to end the life of one person.
Buffy & Angel later in their shows both have shown growth, Buffy said she'd sacrifice Dawn for the greater good, & Giles was pleased with that. & Angel in S4, he got to the point where his team's lives are more important than the other citizens, because lets face it, if they die, mankind is enslaved. again both choices are wrong, but they have to see the big picture.
the humans are governed by the human law, but some humans are above the law, Lindsay, Lilah, Ben-Glory...etc, they cannot be dealt with by the human law, letting Lilah live for 4 years had great consequences, one of them was having his son live in one of the worst Hell dimensions for 18 years, the death of everyone involved in any of her projects. & Lilah knows he doesn't kill humans, thats why she's not scared of him, she does what she wants, play with people, kill people, mess around, no matter what she does she has a get out of death card, & the law can't govern her, so how can someone stop what she is doing?
it was shown in the show that Buffy being willing to sacrifice Dawn was a positive change, & later Cordy came back from a coma willing to torture eve, & Angel told Harmony to eat Eve if she moves.
so Buffy, Angel, & Cordy's words/actions were wrong?
imo, no, it was the right thing to do, the heroic thing. if one murder is wrong, it would be Lindsay, Angel felt he had to, but this murder is the wrong one, Lindsay showed some signs to change, but Angel asked him before if he wanted to run W&H, & Lindsay said yes, but that still doesn't mean he won't change, unless Angel had another good reason, I see this murder as wrong, & only this murder.