When Zackary Kralik was human he tortured and killed a dozen women until he was committed to an insane asylum. As a Vampire he was used by The Watcher's Council as a test for Buffy. They would weaken her, then place her in an area with him, where she would have to kill him using her wits and brain. Unfortunately, he escaped from his cage and killed two people, turning one into a Vampire. Buffy still faced him, after he kidnapped her mother. She tricked him into drinking a glass of holy water which killed him.
We learned a lot from Kralik about vampires, such as what happens when they drink holy water, and a little bit about how vampires are sired. Kralik is like The Gorch brothers, as he seems to have retained a lot of his original human personality. He was probably turned into a Vampire because another Vampire was amused by his human deeds. It seems a lot of motivation for siring comes from what the person did as a human.