To say that he wasn't a good many may be the understatement of my lifetime. To trap an immortal in the body of a child is... It may be the most evil thing I have ever seen. It was hard enough for me for some years, to learn how to talk, walk, how to wear my clothes and hair, to appear older, to gain authority. And I was always tall, strong, and what they would call a tomboy now.
Then, there is the hope you would age again until you were fully grown. I am not nightlocked, or dependent on blood. I can get by quite well on what any human eats. I am not affected by Holy things, do not have to be invited in anywhere. I am stronger, faster, and much harder to kill than most vampires. I am still the millennium-old monster in the human body, but I control that monster, and not the other way around. As to why I would give this to you? Ultimately, because it is the right thing to do. Evil should be undone whenever possible.