"Buffy is important in part because of the increasing moral complexity of its universe. The character Angel clearly contributed to the building of that complexity by being a vampire capable of good. However, since Angel is good because he possesses a soul, he still represents an essentialist definition of good. Spike owns no human soul, yet repeatedly does good; if he can be seen as capable of change, capable of good, capable of love, then he can represent an existentialist definition of good.The chip which prevents him from harming humans can be paralleled to psychiatric medications which allow sufferers a respite and the chance to work through their psychological issues. The subsequent change is thus not simply physiological.

The change may be indicated in part by the metaphor of light and dark in the series. Just as coming to know the night within herself may help to save Buffy, so too Spike may need to come into the light of day. In "After Life," just before she confesses to Spike (and only to Spike) that she thinks she has been in heaven, Buffy comments with surprise on his being out in the "daylight" yet, as he says, "not on fire? Sun's low enough; it's shady enough here..."

"Every Night I Save You": Buffy, Spike, Sex and Redemption by Rhonda V. Wilcox. certainly an interesting read, available with other essays here.