It all has to do with this Season's big bad, the First Evil! (remember 3.10 "Amends") ... He can morph into any dead person/being. He visited Spike, Willow and persumably Dawn, he he uses the same tactics as Spike and Adam in 4.20 "the Yoko Factor". We don't know who is killing the Slayers-in-Training, may have something to do with the First Evil. Cassie Newton was the girl that died in "Help". And as for Spike, we don't know yet, but his chip may be gone or malfuctioning.