(Spike's biggest "error" perhaps is thinking he can somehow "control" Buffy)

I think a big error Spike makes (not sure it's the biggest) is in trying to persuade Buffy that she belongs with him because she is a Wrong Thing like he is, instead of asking himself whether he might somehow belong with her, by becoming more like she is (a champion).

I can see why he would make that mistake. For one thing, Spike wouldn't ever, ever want to imitate Angel, and as far as he could see, Angel had the vampire/champion turf staked out. For another, while William saw himself as a Good person he was bullied and derided, then for the next 100 years Spike's self-esteem had been built around being evil.

Even after all the changes he made: helping the Scoobies, becoming Dawn's protector, it wasn't until the externally-imposed "goodness" of the chip was removed (at least in Buffy's case) that Spike regained the full freedom of good vs evil, chose to hurt Buffy, and then was forced to confront the consequences of his choice... at which point he was ready to contemplate the drastic voluntary surgery of regaining his soul.

(Now I only wonder why it took me so long to find it.)

I know why it took me so long... the title. Buffy the Vampire Slayer?? It sounded like the most awful fluff.

It wasn't until all my friends started squeeing about this movie called Serenity that I investigated... and discovered Firefly... and wished so badly there was more... and realized the guy who made Firefly was the guy who created BtVS... and finally, reluctantly, tried it. I'm so glad I did.