In the Buffyverse, when a human is turned into a vampire, he instantly loses his soul, which is replaced with a demonic spirit. As a consequence, vampires are completely without any conscience; any sense of right and wrong. They are pure evil and not only will not do good -- they are actually incapable of doing good. Such was the clear case of Angel, who mercilessly hunted, tortured, and fed upon humans for 150 years. His bloody rampages continued until 1898, when, after he killed the daughter of a Gypsy mage, was cursed with a soul. Suddenly, Angel was flooded with the moral awareness of what he had done for a century and a half, and was crushed by guilt.
Such is the (semi-) Calvinistic anthropology in the Buffyverse. Vampires are Totally Depraved. They are not able to do good; not able to have the slightest spark of moral awareness. It is only the elect who are given the grace to know good from evil and be saved. Angel represents those who are delivered from the abyss by Limited Atonement, for only a handful of vampires have souls.
A different case is Spike. My wife describes him as "totally hot", but that's a discussion for another day. Spike was a failed English poet who was sired as a vampire in 1880. Spike was powerful and ruthless, and left a flood of innocent dead in his wake. But unlike other recently-turned vampires, he did not immediately slaughter his own family. Instead, he sired his mother in order to prevent her death from tuberculosis. And although he was a hated foe of Buffy and her gang in seasons 2-6, in the seventh season, he fell in love with Buffy. She rebuffed his advances, and in response, Spike tried to rape her. Horrified at his own actions and aware of his inability to love Buffy properly, Spike underwent a grueling ordeal in order to win his soul back.
This is a more Wesleyan anthropology. Spike, though a vampire and thus Totally Depraved, has some moral awareness, although he repeatedly rebels against it. This is the prevenient grace of God which is extended to all people, calling them back to their originally ensouled selves.
Of course, Whedonistic theology is discerned only with limited evidence, and it would also be possible to read Spike's re-soulment as a semi-Pelagian process.