So, Doc worshiped Glorificus, and he knew (since Forever) that Dawn was the Key - but he didn't bother to tell Glory, who kept looking and making a fool out of herself, torturing a vampire for nothing.
Glory found out two episodes later. It's not inconcievable that Doc found out seperately. He didn't know where his Goddess lived, we never see them together so how would he find her to tell her.
He worshiped Glory and was doing his job.
No, his job was some sort of underworld dealer. His religion was separate from that. He had a job that wasn't being a scabby minion- he was an established Hellmouth character than Spike knew of. Most likely he'd been there for some time, i.e before Glory came.
Are you saying that the whole Glory-Key-sacrifice in the tower thing happened because Spike and Willow (she provided the resurrection book) helped Dawn deal with black magick?
Yes, in part. Unforseen consequences to messing with dark forces. Spike says it himself. That's why Spike feels so much guilt.
Unforseen consquences does not equate actual responsibility. Which is why it was tragic/unfortunate.
Besides, only the mentally ill could recognize the Key; Glory herself had to conjure that big snake - and the spell to do it was a very rare one. Doc was a mere demon, I don't think he would recognize the Key just by examining a strand of hair.
When Doc pulled Dawn's hair he said she had a “strong DNA”, which could mean that he figured out that she was related to someone powerful, even making the connection to the Slayer; later, when Xander and Spike went o talk to him,
No just the mentally ill, but also 'second-sight blokes'- monks for example. Rare and unique beings. Doc certainly qualifies.
So he he recognised who she was. He went from kindly old man to creepy demon looked at her with interested black eyes. Why else would he do that? Freak her out for no reason? No, he knew what she was and her significance.
It's only in hindsight that the viewere realises that.
SPIKE: Wonderful. But not why we're here. Hell-god type. Name of Glory has gone missing. She's brewing up some major-league bad, and she's nicked the Slayer's kid sister in the bargain.
DOC: Hmm. That girl you brought here. Sweet little thing.
Doc was lying to them. He lied to them mere moments later when he tried to send them to China for info on The Beast.
So Spikes now being blamed for Buffy's death.
Not blame, unindended consequences. There is a big difference. If you don't see it then whatever.
I don't blame Dawn and I don't blame Spike- it just happened. It was tragic.