This seems to be a common opinion around LJ. And I completely disagree.
For the most part, it’s all about The Initiative. An episode in which Spike is captured by the government, and then escapes. Why does he? Because the Initiative are incompetent… and kinda evil.
To the Initiative, there are two ways to classify things: human, or hostile. Either you’re a normal human, or you’re something nasty that should be locked up or killed. At most, it might label you according to species – Polgara, vampire, werewolf – but then they all get treated the same way regardless of species. The only distinction that really matters is the aforementioned human or hostile.
Buffy and Riley discuss this in New Moon Rising:
“You sounded like Mister Initiative. Demons bad; people good.”
“Something wrong with that theorem?”
“There’s different degrees of…”
They don’t see a distinction between Random Snarly Demon, and Oz. Both exist to be experimented on.
(Also – they’d have no problem killing Clem. Or Lorne. See? I said they were evil.)
This attitude doesn’t just make them evil. It also makes them incompetent.
In Doomed, we get to see the Initiative dealing with a new threat:
“Three metres tall, approximately 100-120 kilograms, based on my visual analysis.”
“Unknown. Probably nothing we haven’t handled before. There is no pattern we can discern yet, so we got to assume that it’s on a basic crush-kill-destroy.”
Whereas the Scooby Gang identify it as a Vahrall demon (“Slick like gold and gird in moonlight, father of portents and brother to blight; limbs with talons, eyes like knives, bane to the blameless, thief of lives.”) and conclude:
“This thing isn’t digging up the bones of a child for fun.”
“Well, a demon’s got some pretty hilarious ideas about fun.”
“Bones of a child, though. I saw that. An ancient ritual – uses the blood of a man, the bones of a child, and something called the Word of Valios. It’s all part of a sacrifice – the sacrifice of three.”
“Let me guess – ends the world?”
“Well, yeah. It’s not big with the details, though. It doesn’t say how the world ends or what the ritual entails exactly.”
“The sacrifice of three… three people are going to die?”
“No, they won’t. Because claw boy is not getting all of his ingredients. We have to find that third one, the Word of Valios, and keep him from getting it.”
The Scoobies have a plan to save the world – while the Initiative are busily running around town, looking for a “pattern-less” demon to lock up in the cells. If they were the only ones on the job, the world would have ended right in the middle of season 4.
The same goes for Spike. If the Scoobies hear that Spike is in town, they instantly drop everything and start focusing on stopping him. To the Initiative, though, he’s just hostile; vampire; number 17. They lock him up, forget about him – and because they don’t realise who they’ve captured, they don’t realise how much they need to watch him. So… he escapes.
Buffy, on the other hand, is not incompetent. Or evil. So when Spike ends up on her doorstep, she evaluates him individually, assesses who he is, and decides the best course of action – which is definitely killing him. Or at least letting him die:
“What part of ‘help me’ do you not understand?”
“The part where I help you.”
“Come on, I’m par-boiling out here.”
“Want me to help make it quicker?”
“What are you saying?”
“I’m saying that Spike had a little trip to the vet, and now he doesn’t chase the other puppies anymore.”
“So you haven’t murdered anybody lately? Let’s be best pals!”
Buffy has no particular interest in keeping Spike alive. But she doesn’t tend to kill demons who don’t pose a “deadly threat to humanity”. She doesn’t like Spike – she hates him – and she knows how evil he is, but right now, he’s not a threat. So she’s happy to let him die, live, or anything really, as long as it doesn’t interfere with her Thanksgiving dinner.
But then… Spike makes a deal with her.
This has happened before. In fact, in one of my fanfics, I called Spike the “Alliance Forger”, because he is constantly making deals with Buffy.
The first one was way back in Lie To Me, when Buffy and Spike made a brief “let the stupid vampire worshippers go, and I won’t kill your girlfriend” deal. And then there was the alliance in Becoming, and the temporary truce in Lovers Walk. Spike has no problem temporarily being on Buffy’s side – and neither does she, actually.
Spike has gone back on his word quite a few times (mostly by violating the “get out of town and never come back ever” clause); Buffy never has. If she’s made a deal, even with her worst enemy, she sticks to it.
So she sticks to this one. The deal is basically: Spike gives information on the Initiative, and in return, Buffy takes Spike in (temporarily) and gives him blood and so forth. And definitely doesn’t kill him… for the moment, at least.
Buffy doesn’t go back on her word.
It’s meant to form a contrast. The Initiative, who are increasingly portrayed as the bad guys, are seeing vampires as impersonal objects, to be given numbers rather than names. And at the same time, the Scoobies are hanging out with a vampire who is definitely not just a number. As Willow puts it:
“It’s ookie. We know him. We can’t just let him poof himself!”
The good guys might kill vampires – but they also know them. They don’t just see mindless beasts roaming the town. They see an organised group, who need to be monitored – who need to be researched. They need to know who the leaders are, and what they’re planning. They need to know where each vampire stands in the pecking order, and how likely they are to have powerful friends backing them up. They need to know when the next apocalypse is likely to occur.
For the Scooby Gang, the “hostiles” are very personal.
Once the deal is made, Buffy sticks to it to the letter. She frequently jokes about killing Spike, but she never does.
In Something Blue, she tells him:
“Oh, make a move. Please. I’m dying for a good slay.”
And yeah – if he went for her, she’d kill him. Happily. But for the moment, the truce still holds.
(The same thing happens in Out Of My Mind. Buffy’s observation on Spike getting the chip out is “That means I’m allowed to kill you.” And she certainly plans to kill him – once she’s allowed to again.)
So, what does Buffy get out of this? Information.
Spike has information on the Initiative, on Adam… He knows what Dracula is like, where Harmony is living, and how the Slayers died. He’s very useful. And he’s making Buffy better at what she does.
The Initiative will always be incompetent, because they’re flying blind. They see a demon and go “Hostile, six feet tall, big claws.” Meanwhile, Buffy knows its name, purpose, and hideout – and now she’s got even more info, because Spike’s around and willing to share (for a price).
Killing Spike wouldn’t just be welshing on her side of the bargain. It would also be giving up a valuable resource.
In short, there are a number of reasons why killing Spike might be a good idea. But something Buffy would do? I just don’t see it.