Apples and Oranges here mate.
Spike - The explanation is right on the page of this comic. She won't hold a person with a soul accountable for actions committed while soulless. Forgiveness isn't even applicable.
But Buffy's trust in/towards Spike has always been muddled. In the latter parts of S5 and 6 she trusts him to protect Dawn, to help out, she considers him an ally- at least one of nessesity.
Then in S6 they enter into an abusive relationship, Buffy's feelings for him swich from hate to twisted affection and back- and then he tries to rape her. She stops him and he flees and because of this he goes and gets his soul.
And after he comes back in S7, he's mentally unstable, murderous (he kills a dozen or so people and turns them in some sort of fugue state) and unable to control his darker urges.
This is when Buffy starts to trust Spike?
This isn't years that have past, this isn't 'I've changed and become a different person in my behaviour/outlook', it's simply blind trust because he has a soul now.
It's never been a clear cut case of 'he's good now and I can trust him' ecspecially since you've pointed out how Buffy has met plenty of people who happily do horrible things with a soul. One of them's her best friend.
I have big issues with the whole 'Spike is a completely different' person now, since I've never been able to see one single difference in the way he acts/talks/relates to other characters. There's just no evidence for it. He loves Buffy pre-and-post soul, he snarks with Xander pre-and-post soul.
Because of the poor writing Spike remains, in every way that matters, consistantly the same character he has been, except with his more extreme attributes toned down.
I still have a problem with the way Spike sees his pre-ensouled state as something to be proud a lot of the time, and ultimately he views himself more as sinned against that sinning.
Willow - Best friend of many years. Possibly the person she is closest to. Just important to remember for context. She lost her partner and then wanted revenge on those responsible. She made some awful decisions but the worst of them were influenced by dark magic and what Giles did to her. It was never personal and there were some extreme mitigating circumstances. Rightly or wrongly it's easy to see why Buffy is willing to forgive.
Willow (and it was Willow- high on dark magic and vengeance she had pretty much all her mosr unpleasent personality traits, plus tics) tried to destroy the world out of pure selfishness- because she couldn't live with the pain.
She also tried to murder Dawn, Giles (plus she tortures him) and Buffy herself in cold blood.
Three months and its all okay? Please.
Giles - In Helpless Giles betrays Buffy in the worst way. He drugs her and leaves her for dead. Buffy sees Giles as a father figure though and forgives him almost instantly. It's a case of once bitten twice shy though. He abandons her when she needs him most in Season 6 and returns in Season 7 and attempts to kill her friend and ally behind her back.
Giles was clearly torn in his Watcher duties in S3 (we know thet, barring a rebellious phase, this way of life has been part of his family and all that he's known since he was a young boy- he was groomed for it) in fact you could argue that Gilkes was programmed to a certain extent. Still, in the end he tells Bufyf the truth and saves her from a vampire attack. (Lots of people forget that last part, when he risked his life to save her)
From then on he has no obligation to Buffy, he's been fired, yet he remains part of her life in S3 and is crutial in supporting a lot of her decisions.
After that he's pretty much Buffy's rock in S5, helping her cope with the loss of her Mom, helping with Dawn. In S6
fans may hold a grudge for leaving Buffy, but by the end Buffy herself doesn't. 'You were right to go away.'- so I doubt that she is angry with him over that.
The 'trying to kill Spike' thing, yeah, that was wrong in Buffy's eyes, no question, but it doesn't even compare to the crimes of of Spike- while soulled, and Willow did.
It's mostly just plain hypocracy.
Faith - Buffy has known Faith for all of a couple of months. They have a budding friendship but no genuine bond to speak of. Then Faith, while fully souled and under no magical influence, commits murder, rape & torture. She attempts to kill everyone in the town where Buffy lives. She also wages a personal vendetta against Buffy. Targets include both her best friends, her mum, her boyfriends and herself. That Buffy can even look at Faith again is a real show of how forgiving she can be.
Genuine bond? How about being the Slayer, which is the big thing in Buffy's life that seperates her from everybody else except Faith.
Faith did all those things you said (apart from the rape but we've already argued about that) with full awareness and complicity- but a strong argument could be made for mental instability/trauma after the Finch incident.
Add to that an abusive childhood (hinted at but it's confirmed in the comics), that fact that she's a 16/17 yr old when the crimes took place, major trust issues with pretty much anything alive and the guiding evil paternal hand of the Mayor, these are all mitigating factors, they don't excuse everything she did, but they do put it in a moral context.
Faith, like Buffy, is also infested with demonic energy, part of the slayer package. They have the urge to kill in them right from when they're called, to some extent they get off on the violence- its in their nature to enjoy it. Buffy denies it but it's evident in her character from S2 onwards.
They are both touched by something darker, and without a balanced approach to slaying it can lead to unstable behaviour. Casual killing, emotional disassiciation, sleeping with skeevy vampires.
So it's not like Faith is completely untouched by 'magical influence', it's part of who she is. And from S4 onwards she's done nothing but tried to make amends for the things she's done. That's evident in her character and her behaviour, it's not just talk.
Plus Faith tried to kill Angel three times and their pretty close pals these days (ever since S7 anyway) and she tortured Wesley for hours and yet Wesley willingly worked with her when nessesary. She and dark Wesley even seemed chummy by the end of 'Orpheus'.
Or maybe that's just because Angel has had a more adult view of the concept of forgiveness, and the moral greyness that lots of these characters exist in.
The issue isn't whether Buffy should or shouldn't forgive these people. To forgive somebody is a noble act, just as trying to make penance for past misdeeds is also noble, but its hardly as clear cut as you laid out.
This isn't apples and oranges, more tangerines and satsuma's.
Some consistancy on Buffy's part would be nice.
Thanks! :)