"So there's this thing called the Cruciamentum that apparently the Watcher's Council does to Slayers when they turn eighteen that involves drugging a Slayer to lose consciousness and also her powers, locking her up with a vampire in restraints timed to come off when she wakes up, and see if she manages to kill it.
This would have been an unpleasant enough surprise if it weren't the case that my Watcher fucked up the dose of knockout drugs, and when I woke up the vampire had gotten out and managed to kill my parents and twin sister. I waited until the power-suppressing drugs wore off, tracked him down, ripped his head off, and confronted my Watcher. He was appropriately penitent for the misdosage but very indignant with me for objecting to the test. I killed him a lot more slowly than the vampire, and then obsessively Slayed my way across the continent for the next two years before running into a vampire I couldn't handle, who decided to turn me.
On waking up without a heartbeat, I was freed from the crippling grief of the last two years, but still incredibly pissed off, so I hunted down the Gem of Amara--I wouldn't normally be admitting this to the Slayer but, uh, walking around in broad daylight, bit obvious--took a plane to England, slaughtered the Council, and began hunting down stray Watchers. When I found him," she gestures to Giles, "I discovered that he had excellent protections, which prevented me from killing him long enough for it to be established that he thought the Cruciamentum was abolished centuries ago, so I had no reason to."