If your first foray into Star Wars was a children's book, you probably had never heard "darth" before and may have thought the author made a typo.
*kid watches Star Wars*
Ben: ... Darth Vader, who was a pupil of mine until he turned to evil...
Kid: WTF does "darth" mean?
A type of person who is sly, shady and dark towards their so called "friends", hence contracting the name Dark Vader
DAVE: errm errm na errm, i'm really tired, sorry i have to go now... *hangs up*
(Dave is on the way to the pub already)
BOB: *sends text message* - AHHHH HAHAHA YOU LITTLE DARK VADER! I HOPE U DISAPEAR INTO THE SHADOWS DARK ONE!