Countless vamps and fate decided that there only needed to be one slayer in every generation. [Which wasn't strictly true anymore, but that would require too much gabbing on her part.]

Normally, a vampire has no soul. Angel was an exception--he's a vampire who was given a soul. Normally, he's one of the good guys. But thanks to the this experiment, he's become the vampire he used to be, pre-soul. Angelus--not the happiest of campers. The sooner I can find Angelus, the sooner I can stop him from hurting anyone else.

Once Angel's back to his less-evil-more-broody self, he'll be feeling a killer cocktail of regret and guilt over all of this.