Hmmmmm. I think you are right, and I'll bet that scene was put in there to create the space for Giles to "go bad" if they chose to do so later. However, WTTH is all about subverting expectations, so cute schoolgirl is the monster, creepy old guy is the helper, and mysterious guy in the alley is...neither. I'm just as glad that they went the way they did. ;-)

Giles turning into the big bad — through hubris and for the greater good — could work really well. Certainly, they could have used Ms. Calendar's death as the catalyst. I do worry that readers would find it bash-y though. One of the valuable things Giles brings to the story is that of the "sadder but wiser" adult, who made many of the same mistakes Willow did, but was able to overcome his infatuation with power and channel that into a dedication to service. Plus, I guess they needed a "teacher" for the mythic structure to work.