A Sympathetic Vampire is a vampire who considers being a vampire a "condition" and loathes having to be a vampire. Whether or not they are still malicious depends on telling, as being a sympathetic character does not affect whether the character is good or evil.
The figure of the Sympathetic vampire was introduced in Varney The Vampire.
Sympathetic Vampires are used in the way their title suggests; to make the reader or veiwer feel sorry for the vampire, who is portrayed as a tragic character.