The Whole Sire Thing…
Basically I've been trying to look at the problem from a story POV. What were the writers trying to do and why?
Let's look at 'School Hard' first, and see what Angel says about Spike, keeping in mind that none of them has heard about him before:
Giles: Well, he can't be any worse than any other creature you've faced.
Angel: (suddenly appears) He's worse. (they all look at him) Once he starts something he doesn't stop until everything in his path is dead.
Giles: Angel, do you know if this Spike fellow goes under any other name?
(They all look where he was, but he has disappeared. The library doors finish shutting.)
Well that was spectacularly unhelpful. Angel really does have a thing for cryptic warnings, but this time he outdoes himself. The only useful thing he says is that Spike is very dangerous—and Giles figures that out all by himself later, when he discovers that Spike has killed two Slayers. Which leaves the question: Why does Angel not say any more? He obviously knows who Spike is, but doesn't explain how—has he met him? Fought against him? What sort of info does he have and why does he not tell them anything they might need?
Later, when they're being held captive in the school, Giles sends Xander to get Angel. And now we find out why Angel has kept so quiet:
(Cut outside. The man lies dead on the grass. Xander and Angel see him.)
Xander: You know a lot about this Spike guy, so, um… you got a plan?
(Angel grabs Xander by the throat.)
Xander: Good plan.
Angel: I taught you to always guard your perimeter. Tsk, tsk, tsk. You should have someone out there.
Spike: I did. I'm surrounded by idiots. What's new with you?
Spike: You think you can fool me?! You were my sire, man! You were my… Yoda!
Angel: Things change.
Spike: Not us! Not demons! Man, I can't believe this. You Uncle Tom!
Angel knows Spike—but not as a foe, oh no. The important words here are 'Sire' and 'Yoda'.
'Sire' in this instance has nothing to do with who gave William a neck trauma—sire signifies (close) family. (Remember, I'm trying to see what the writers are telling us!) It means not only do they know each other, they are related. The last close relative of Angel's that we saw was Darla—and she came pretty close to killing Buffy. We see the same with Spike. Angel's family is old, dangerous and well-trained. They're not your average stupid fledgling that Buffy comes up against most nights—they're a serious threat.
'Yoda' is to denote the relationship between the two—teacher and pupil. If Spike is dangerous, it's because Angelus trained him. Angelus was the one who made him someone who 'doesn't stop until everything in his path is dead'. What Spike was like before he was turned doesn't matter and is never mentioned—it's who he became that's important. Because if the pupil is deadly, then how much worse is the teacher? This of course we find out halfway through the season when we finally meet Angelus…
Now 'Fool For Love' is a very different sort of episode. It's one that fills in Spike's background, and does so superbly. That the writers decided to make Dru his Sire (instead of Angelus) makes an awful lot of sense, especially since we finally see what sort of man William was. Angelus would never have noticed him, but Dru looked into his mind and saw what was hidden from others. And she made herself a gallant and caring knight, who spent 120 years at her side, watching over her with affection and tenderness, even as he carved out a name for himself as a vicious fighter.
His relationship with Angelus was obviously a tumultuous one, but it stands to reason that it was Angelus who taught him—Dru is hardly the type to impart lessons. And although he was rebellious, we see that Spike did pay attention to what his (grand)sire was telling him:
Angelus (1880): A real kill. A good kill. It takes pure artistry.
Spike (2000): Death is your art. You make it with your hands, day after day.
That is the importance of the Spike/Angelus relationship—Spike arrives in Season 2, setting himself up as The Big Bad, but he's not. He's only the forerunner for the Real Big Bad—a taster as it were. The teacher/pupil unevenness is already at play. Angel always seems to have the upper hand—especially when it comes to the women in their lives. Both Dru and Buffy in some way lost their hearts to Angel, and Spike doesn't have a hope of ever getting there as far as we can see.
Which is why what might to some be a ret-con really does not bother me at all. The stories, the relationships, the interactions do not change because of it. In fact, it makes them more. Spike's affection for Dru suddenly makes more sense—his devotion to her is bordering on the selfless, even though she is insane, difficult and (when we first see her) ill (Angelus would probably have abandoned Darla in similar circumstances). But from the moment Dru snatches 'Effulgent' out of thin air, we can see that William is utterly smitten, and it is obvious that he'd dedicate his unlife to the woman who first understood him (as he so eloquently explains in 'Crush').
And it is because of things like this that I love the show. :)