When I completed "Many Loves," I got a barrage of complaints from redemptionistas complaining that Spike was not inherently evil.  Which pissed me off, because I never said he was inherently evil.  What I said was: 
 

My argument, therefore, is that Spike is neither inherently good nor inherently evil.  A lot of people can't seem to grasp this concept- don't you have to be in one camp or the other?  But consider human beings.  Most of us are inherently "good"- concerned for the welfare of humanity as a whole, with varying degrees of selfishness and unconcern thrown in.  Most of us, if thrown on a Hellmouth, would turn out to be Scoobies, or at least any nameless Sunnydale High student, afraid to fight evil but trying not to add to the problem.  But some of us would be Faiths or Ethan Raynes or Holland Mannerses- destructive and self-destructive out of fear, anger, selfishness, or greed.  Try to convince me that all humans are concerned with the well-being humanity as a whole and I'll have a few examples from history to counter you with.

Truth is, most people don't take global welfare into consideration when we make decisions.  We want to protect ourselves and the ones we love.  Look at Buffy's problematic decision to sacrifice the world, if need be, to protect her sister.  Take these common misconceptions about the events of Season 2:
 

There's a flaw of logic here- the idea that a "good" or "evil" result means a "good" or "evil" person performed the action.  Spike's motivation is not one of good or evil, but of love.  His character is driven by emotion and loves intensely and compulsively, and this emotion affects all of his decisions.  This quote from James Marsters at GenCon: 
 

Redemption is an Angel-question.  Liam was a jerk, Angelus was a bigger jerk, and Angel is a guy who is sometimes a jerk but trying very hard not to be.  He's changed.  Spike hasn't changed; he's just redirected his energies.  His actions of the last season are pro-good, but if Spike is acting in the interest of good, he's doing it to please Buffy.  It's not as simple as impressing her with his actions, although that's also a factor.  He's trying to be what she wants him to be.  In short, Spike does want to be good, but not for the sake of goodness itself.  For instance: he didn't feed off the bleeding disaster victims because he knew it would piss her off.  And he expected credit for that- his willingness to give up fresh human blood to please Buffy.  Because in Spike's mind, the "right" choice is the one that garners affection from the object of desire.  She, being inherently good, wanted him to give up fresh human blood for the sake of the fresh humans.  This was clearly not what he had in mind.  Even his more noble actions, like the ass-kicking he took from Glory to protect Dawn, don't imply redemption.  Selflessness doesn't equal goodness, because selflessness often differs according to who the receiver of the selflessness is.  Yeah, I know people who are considerate and giving to everyone in the world equally... I'm not one of them.  I'm nice to my friends and polite to everyone else.  Usually.  Raise your hand if you're nicer, more patient, and more giving to your boyfriend/girlfriend/husband/wife than you are everybody else.  Yeah, that's what I thought.

But if Spike is not inherently good, or inherently bad, where does that put him as a soulless vampire?  I would argue that being a vampire is not so much a question of inherent evil as one of inherent violence.  Violence is a force that can be used for good or evil, even if it's usually associated with evil.  In case you haven't noticed, Buffy is very violent by nature.  She uses her violence for good, but the violence is a fundamental part of her nonetheless.  She tends to hit things when she's frustrated and use force when she might not necessarily have to.  Spike's current quasi-Scooby status serves a twofold purpose; he gets to kill things and get on Buffy's good side.  Give me one example of Spike wanting to rid the earth of vampires as Angel does- simply for the sake of ridding the earth of vampires.  Go on, give me one instance of that.  Spike doesn't care about good and evil.  He wants to hit stuff.  He waggles his tongue when he stakes vampires.  He gets off on it.

Redemption is Angel trying to compensate for his crimes.  Redemption is Lindsey leaving Wolfram and Hart because he can't stomach the idea of killing children.  Spike is not being redeemed.  Unlike Angel, Spike does not regret his crimes or wish to regain his humanity.  Like Anya (who is somehow "good" although she caused havoc for ten times as long), Spike speaks of violence past with relish and nostalgia.  (Anyone else remember her effusive description of vengeful accomplishments to Xander in "The Prom," or munching popcorn happily through Giles' gory "lecture" in "Hush"?)  He isn't rid of his violent, vampiric nature, he's just trying to cope with having to rechannel it.

And, finally, the fundamental question: why does everyone want Spike to be good, anyway?  What exactly would that accomplish?  Would being "good" somehow validate him as a character?  Would you stop feeling guilty that you like him so much?  Do you want him to be the kind of guy you can bring home to Mom?  Because I sure as hell don't.  Spike is easily one of the most interesting villains in Buffyverse history, and he's never so much fun as when he's making a mess. 

Imagine, if you will, a "good" Spike.

He'd brood about his past misdeeds.  He'd sulk and mope and try to make amends.  He'll kill demons not because it's fun, but because he feels obligated.  He'll stop having UST with everything he hits.  He'd stop smoking and wearing leather, 'cause only evil characters do those things.  He'd be stalwart and true and utterly dull.

No, thank you.

Resources:
Redemptionist: The Bloody Awful Poet Society