Who's the most irresponsible mentor of all time?S

We all need a little guidance sometimes. So it's a good thing there are mentors, who can teach us and shelter us and... generally screw everything up. Because that's what mentors do. But who's the most irresponsible or just plain evil mentor in science fiction or fantasy?

Please include a picture or clip (or book cover) for the great teacher of your choice. And please mention his/her name, where he/she appears, and a bit about why you can never, ever, trust them. Thanks!