Well, Buffy blames herself almost entirely, because that's Buffy, but Spike was no angel in Season 6, either. He was trying to make Buffy need him by pulling her into the dark, not knowing that his desperation, anger and frustration were helping to shape what could have been his child. I think he's equally horrified that Monster reflects some part of his love as well.

The last line is meant to evoke the possibility. I suppose it all depends on what you consider to be Spike's home at the end of this fic.