
Twigs |

You could steal "Vamp" straight from the Metal Gear series. "The Leech" has potential... but I have my misgivings.
Honestly, I don't see this as cause to be called anything but "witch". This is the kind of magic that terrifies the narrow-minded folk and breaks more than a few unwritten rules. Nicknames are usually a form of endearment. Personally I'd find it hard to be endeared to somebody that grabs people by the throat and steals the very life from them on a regular basis, and the kind of people that do are probably too busy binding you to the sacrificial altar. It depends on the tone of your game, and I really don't know it and don't want to seem like I'm telling you what to do. I'm just suggesting this is something most people won't fully understand, and you should come up with something ignorant and narrow minded that reflects that. That's my advice.
... come to think of it, we did once have a "Witch Boy" at our table.
Edit: Disregard the above. Spookyfingers is the only right answer :P.

Ravingdork |

Are you looking for serious, or humorous?
I'm looking for a serious nickname (something that would make the little people awe in fear). Something that would be attached to a name (which I haven't decided upon either), like in the old Batman serials or in fictional mobsters, like Cid the Squid or Manny Mancatcher or Benny the Leach. (But more sinister and wizard-like if you please.)
However, I enjoy the funny ones too.