So I come at this from two places 1) I love the potential of Spuffy but hate almost everything in s6 and 2) Buffy is never wrong. I mean never.
I love love love so much the way she confides in him about being in heaven. the way she goes to him when she can't be around the others, the way he has kept count of how many days she is dead and gently takes her to the sofa to fix up her hands - while her friends burst in all noise and confusion and demanding things of her. I love everything between them from the end of 'Intervention' when she kisses to him to the end of 'OMWF' ... when she kisses him.
After that kiss, though - things start to change. She made a mistake and admits as much. But he thinks he is making progress and won't let it go. He comes to her when she is another vulnerable moment and she makes the same mistake again and again. The next time she sees him she tells him it was a mistake but he doesn't let it go.
Now I don't particularly blame him for that. Hope springs eternal and all. Just because he wants it to mean more than it does doesn't make him a bad person (though the fact that he is a bad person makes him a bad person) and being pushy and pressing his point is forgivable because people really do that. They want what they want and they try to get it. Of course, in his case he's only getting any success because she is in such a low place - and I would expect a human to recognise that, and judge them for not backing off - but Spike isn't human, he doesn't have a soul - I get it, he doesn't get it. That's not technically his fault.
Neither is it her fault for continually making the same mistake. She's incredibly low, he's there wanting something of her and she gives in to make herself feel better. She always owns up to it and tries to tell him it was a mistake.
They are just working at cross purposes and she is too depressed and he is too soulless to realise that, in the moment (though she always recognises it after the fact).
Once he finds out he can hurt her, though, he changes completely - from being this rather sweet presence, hopefully hanging around her and genuinely wanting to help even if he is actually muddying the waters, to wanting to hurt her. The way he tells her she came back wrong, the way he beats her - it is vicious and cruel and I hate it. I think it stems from him having her on a pedestal, believing she is so far above him and too good for him and then, when he thinks she isn't human, that topples her in his mind and he wants to punish her for acting like she was better than him when she wasn't.
And Buffy is so depressed and feeling so wrong anyway that she reacts to this by letting it happen, by embracing it. It's almost a relief to her that there is a reason she feels this way - and it is because she is not who she was before.
What stems from that is their sexual relationship, where I absolutely balk at the idea that it is mutually abusive. The only abusive act Buffy perpetrates is the extent to which she beats him up in Dead Things. In everything else, Spike is absolutely the abuser. Buffy using him for sex is not abusive - especially when she tells him this time and time again. She never tells him she loves him, she never pretends it's more than it is, she doesn't even pretend she isn't disgusted with herself for doing it. She is completely honest about what this relationship is to her and how she feels about it and him. Spike could absolutely refuse those terms and conditions. The reason he doesn't is because he doesn't believe her when she says she doesn't love him. But that is on him not her. She is honest. His delusion is not her fault.
In their relationship, she turns up at his crypt for no strings sex and is quite open and blunt about the fact she doesn't really like him. He repeatedly turns up at her place of work, where she is feeling her lowest; he follows her home and badgers her for sex when she just wants to get inside and back to Dawn; he turns up on her night out with her friends and whispers horrible things in her ear and has sex with her even after she has told him to stop; he doesn't accept her breaking up with him and he threatens to tell her friends about them unless she sleeps with him again - telling her they would cast her out if they knew.
Their actions are not really comparable. And any attempt to make them comparable just comes across as judgement on a woman for daring to have sex with a man she didn't love.
But then this is where my mind divides in two: between hating what happens in the show and forgiving the character who does it. I think the show definitely tries to make the relationship seem mutually abusive through what characters say, despite that not actually being shown in their actions. That is dangerous and victim blaming. I think it depicts a clear example of rape in Dead Things and acts as if it is no big deal - because it isn't violent. That is dangerous. I think making the attempted rape a jumping off point for his redemption arc is dangerous and in poor taste. I think not showing Buffy dealing with this in any truly meaningful way and instead focusing on Spike's reactions is dangerous and misogynistic. I think trying to keep Spike likeable despite his actions throughout the season was a mistake. I think there are a whole load of frankly disturbing mixed messages that are sent out, made all the worse when it is considered that Buffy's core audience was teenagers and most likely girls, and that the writers were either not skilled enough or not aware enough to make what they wrote OK, and find a way to send out a coherent message that was mindful of who was watching and learning from this show.
However, this is all an extratextual mess created by the writers and I don't blame Spike for it, and I don't hate him for his actions. Because Spike is evil. He is. We love him - but he is. And so I can accept that when he insists she does love him and tries to drag her down into the dark with him he is - in his own way - trying to help. He thinks he understands the darkness inside of her and that she can only be happy once she owns it and starts living that dark life - that she is living a lie with the scoobies and she cannot be happy living a lie.
I think this is equal parts wishful thinking (and I can't blame him for wishful thinking) and a consequence of him being an evil, twisted creature without a soul, particularly one that has started to act against his own nature. He isn't trying to hurt her, he isn't trying to abuse her - he genuinely thinks he's helping (which is him acting against his nature), but his help is the help of an evil, unsouled monster (which is his nature). So of course it is no good
Because Spike isn't more noble than other vampires, he isn't more inherently good and he certainly doesn't have a partial soul, like Fury said he did. What he has is a chip. A behavioural modification chip. And it is modifying his behaviour. Now what it was supposed to do and how it was supposed to work is up for debate, but he had it installed in lab conditions and was supposed to remain in lab conditions. He didn't - and now the experiment has gone off the rails. He isn't receiving any of the added checks or brainwashing he was supposed to receive, but he is still being conditioned by the pain. Not only does it physically restrain him, but it also forces him to change the way he does things to work around the pain. So he has become dependent on the scoobies - and has to help them in order to survive. And that changes the way he acts around them, and how he sees them - particularly Buffy.
I believe he does really love Buffy - like he really loved Drusilla. Vampires are capable of love between each other. They can even remember love for the people they loved in life. But he can only love her - a human he met as a vampire - because of the chip forcing its changes on him. Even so, his fundamental nature isn't changed, it's surface level; changes made in order to ensure his continued survival. Spike doesn't know this, though. He thinks it's real - he says as much in s5 in Crush. He starts comparing himself to Angel, he claims that he can be good. And he is doing good things - and loving a good person. No wonder it seems real enough to him. But at heart he is still twisted and dark and evil.
So when Buffy is in pain, he truly wants to help. He truly tries to help. But he is dark and twisted, so the help he offers is dark and twisted. He doesn't accept her telling him she doesn't love him or that it is over, because he truly believes this is her fighting against her nature and she needs him to show her it's OK to give into it.
...which is how we get to the AR. And from a character perspective I suppose it makes a certain sense - it's just horrible to watch and it would have been preferable if they hadn't gone there.
But in Entropy he tells Buffy of all the terrible things he might be accused of, he doesn't hurt her. Then he inadvertently hurts her by sleeping with Anya and her finding out. So he goes to put it right. And ends up hurting her even worse. Because as a monster he cannot help but hurt her.
And that moment, when he sees the pain and shock and betrayal in her face, and realises what he has done, that is when he realises that he has been wrong about being good, about helping her, about thinking she could love him. That is when he realises the chip's changes did nothing to change his nature and it is that which has just come out, it is that that Buffy rejects and it is why she won't ever want to be with him. It throws him into a full on existential crisis - which we see - he genuinely thought he was different, he wanted to be different for Buffy, and now he realises he's not - and he decides the only thing he can do is change his nature, which means getting his soul. So he does that. And once he has his soul he can view their relationship with much more clarity - and realises things he didn't see even after the AR.
All that is fine, all that is in character and none of it makes me dislike Spike. I also don't mind Buffy drawing a line between Spike with a soul and Spike without, because she has shown capacity to do that with Angel, she didn't hold hyena Xander's sexual assault of her against him ... the entire premise of Angel's show is predicated on the audience not hating the souled version of Angel for what Angelus did. It's only fair Spike is treated the same.
But the way we get there does feel like an irresponsible story line handled badly by writers who didn't really understand the implications of what they were writing. And it does feel like, in order to make Spike seem less reprehensible, they do try and place the blame for everything equally on Buffy. Which is not OK - because she is the victim. But the show goes out of it's way, in this things Spike says, to make it seem like Buffy using him for sex is somehow level with everything he did.
I don't blame Buffy at all for what happens, but the show does and I'm not OK with it. I can still like Spuffy because I can rationalise the victim blaming as bad writing and understand how what happened happened because of the chip - that Spike was still inherently evil, with a wish to be good pasted over the top and even that wish came from a selfish place. And that he didn't fully understand any of that until after he got his soul back.
I wish the show had leaned into that explanation rather than try and minimise his actions with false equivalence. They don't try and pretend Angelus is anything but as evil as he is, and yet they take it for granted that we will enjoy his presence and forgive Angel for everything Angelus does. I don't understand why they didn't want us to see Spike as evil when he was - and just have us be OK with that, as we always had been.
It is the fact that they try and make something morally grey when it is actually black and white that is most troubling to me.