In her answer to another question, Slytherincess mentioned this from a chat with JK Rowling :
From a web chat:
Jaclyn: Did Lily ever have feelings back for Snape?
J.K. Rowling: Yes. She might even have grown to love him romantically (she certainly loved him as a friend) if he had not loved Dark Magic so much, and been drawn to such loathesome people and acts.
What is it that drew Snape so strongly to Dark Magic™ that JK says he loves it?
The other dark wizard's seem to have a fixation on power, and merely use Dark Magic as a means to an end. Even their favouring purebloods is, in part a play for power over the 'less powerful' mudbloods.
So why is/was Snape in love with Dark Magic?