Serial Killer, no. Though I have not read everything on him, I believe as a Witch Finder, his "evidence" would be used in a trial held by the local legal judge or Justice of the Peace and they would find the guilt or innocence of the accused and would have them hanged as proscribed by law. Remember, practicing Withcraft at the time was a crime under the law of the land and the punishment was death. Since it seems he did not have a hand in the actual physical deaths of the accused, he cannot be deemed himself a serial killer. And unfortunately, WT would probably not permit this: Category: Right Awful Rat Bastard.
Actually, he would be said to have been involved in the act genocide, the killing of a large group of people of a specific group. Probably started out as a true believer in that witches exist and all of his activities seem to have occurred in the Puritan stronghold centered around Essex during the Civil War. Then he probably discovered the value of a good Witch Finder in the local towns were willing to pay, and apparently pay well, for such services.
And yes, his Profile belongs on WT. Though not from the good branch of the tree, he was a human being and he did live....sadly.