OK, another idea I'm working out. How does the idea of a villain with a soul change over time in Buffy? First, a list of Big and Little Bads of the soul-having variety--i.e., humans.
Little Bads
Catherine Madison (The Witch, 1.03)
baseball coach (Nightmares, 1.10)
Marcie (Out of Mind, Out of Sight, 1.11)
Chris and Eric (Some Assembly Required, 2.02)
frat boys (Reptile Boy, 2.05)
Ethan Rayne (Halloween, 2.06)
Billy Fordham (Lie to Me, 2.07)
swim coach (Go Fish, 2.20)
Pete (Beauty & the Beasts, 3.04)
Gwendolyn Post (Revelations, 3.07)
Joyce Summers & MOO (Gingerbread, 3.11)
The Watcher's Council (Helpless, 3.12; Checkpoint, 5.12)
Jack O'Toole & Co. (The Zeppo, 3.13)
Cafeteria Lady (Earshot, 3.18)
Tucker (The Prom, 3.20)
Jack (Beer Bad, 4.05)
Faith (This Year's Girl & Who Are You?, 4.15 and 4.16)
Jonathan (Superstar, 4.17)
Mrs. Holt (Where the Wild Things Are, 4.18)
McClay Family (Family, 5.06)
Warren (I Was Made To Love You, 5.15)
Rack (Wrecked, 6.10)
RJ (Him, 7.06)
Amy (The Killer In Me, 7.13)
Big Bads
Faith (season 3)
Maggie Walsh (season 4)
Ben? (season 5)
The Trio (season 6)
Willow (season 6)
My thoughts are all over the board on this. The first thing that strikes me is how many of the Little Bads in the early seasons were actually human. In those seasons, there was an idea of human baddies being taken care of by law enforcement, too--the guys in Reptile Boy, for example. Or they somehow destroyed themselves or were destroyed by their creations (as in Go Fish), which was OK too. Alternatively, they were trapped or dispensed with and not killed, which was an easy way to get around moral ambiguity. The first time we see a Scoobie or proto-Scoobie take out something with a soul, it is the newly returned from Hell and not quite all there Angel killing Pete in Beauty & the Beasts (3.04). Later in the season, Oz-as-werewolf takes out Jack O'Toole, but that doesn't really "count," since he's already dead. By the end of season 3, though, you have Buffy making an honest effort to kill the completely human Faith, which is a pretty radical shift.
Faith is not only the longest running human LB, though, she's also the first human Big Bad, and for the next four seasons, the Big Bads are, at least in part, human. After Faith, though, Buffy's hands don't ever get dirty attempting to kill a person again. Maggie Walsh goes back to the destroyed-by-her-own-creature scenario, Giles kills Ben, Willow kills Warren, and nobody ends up having to kill Willow. The later seasons treat the human LBs differently, too, with them more likely to live on and not be killed or contained. At the same time, we're more and more often introduced to creatures that we'd previously have written off as "soulless" who are still OK. Not just chipped-Spike, but also Clem, and then all of the demons at Xander and Anya's wedding in the sixth season. The lines get significantly blurred.
Which brings me to Caleb. I didn't include him on the list, because I am honestly not sure what he's supposed to be. Human, but more than human? Does he take the place The First had intended for Angel (I just watched Amends, so that thought is fresh in my mind)? When Buffy kills him, it doesn't come off as her killing a person, but does it just not matter by then? Has the not killing people clause in the slayer contract completely run out?